The final list of candidates in the running to succeed Nigel Farage as UKIP leader will be announced later.
The party has been carrying out "one final round" of vetting to determine who is eligible to stand in the race.
UKIP donor Arron Banks suggested Steven Woolfe will not be on the ballot.
Mr Woolfe's candidacy is in officials' hands after he missed the deadline and revealed he did not disclose a drink-driving ban when he stood to be Greater Manchester's Police and Crime Commissioner in 2012.
Also in the race are MEPs Jonathan Arnott and Bill Etheridge, while Huntingdonshire councillor Lisa Duffy and activists Phillip Broughton and Elizabeth Jones have also put their names forward.
There is continued uncertainty about whether prominent MEP Diane James has applied.
The contest has been sparked by Mr Farage's decision to stand down following the UK's vote to leave the EU, saying his "political ambition has been achieved".
UKIP leadership: Final list of candidates to be named
BBC News
3 August 2016
Nigel Farage announced he was stepping down after the UK voted to leave the EU
The final list of candidates in the running to succeed Nigel Farage as UKIP leader will be announced later.
The party has been carrying out "one final round" of vetting to determine who is eligible to stand in the race.
UKIP donor Arron Banks suggested Steven Woolfe will not be on the ballot.
Mr Woolfe's candidacy is in officials' hands after he missed the deadline and revealed he did not disclose a drink-driving ban when he stood to be Greater Manchester's Police and Crime Commissioner in 2012.
Also in the race are MEPs Jonathan Arnott and Bill Etheridge, while Huntingdonshire councillor Lisa Duffy and activists Phillip Broughton and Elizabeth Jones have also put their names forward.
There is continued uncertainty about whether prominent MEP Diane James has applied.
The contest has been sparked by Mr Farage's decision to stand down following the UK's vote to leave the EU, saying his "political ambition has been achieved".
Succession issue
Steven Woolfe
The vetting panel had been expected to announce on Tuesday which candidates were eligible to stand under the party's rules after nominations closed on Sunday.
But after the panel met it issued a statement saying the final list of leadership candidates would be revealed on Wednesday at midday.
"An NEC-led panel has today met to determine the eligibility of those candidates that submitted their nomination papers to stand for party leader.
"That panel has now met and with one final round of checking to go the party aims to announce the full list of candidates at midday tomorrow, Wednesday 3 August," a party spokesman said.
To qualify, candidates are expected to have been party members for two years and to have been nominated by 50 members of the party.
Leadership contest timetable
31 July: Nominations closed
3 August: Final list of candidates announced
1 September: Ballot papers issued to party members
15 September: Votes counted and the winner of the contest announced
Steve Crowther, who left his job as UKIP chairman earlier this week, would not be drawn on whether Mr Woolfe - seen as the frontrunner in the race - should be excluded from the contest for submitting his papers 15 minutes late - a delay that the MEP has blamed on computer problems.
But asked about the issue of the drink-driving conviction - which Mr Woolfe incurred in 2002 but which he did not reveal when he stood to become a police and crime commissioner in 2012 in a possible breach of electoral law - he told the BBC that this was "arguably more serious".
'Coup claim'
Mr Woolfe got a £350 fine and nine-month ban for being drunk in charge of a scooter but insisted his life had moved on by 2012 and the conviction was spent by then.
Amid reports of a power struggle within the party, UKIP NEC member Victoria Ayling tweeted on Tuesday evening to urge her fellow panel members not to "block" Mr Woolfe.
UKIP donor Arron Banks later tweeted: "Tonight's decision to exclude Woolfe is the final straw" and he claimed it represented a "coup" by UKIP MP Douglas Carswell and Neil Hamilton, UKIP's leader in the Welsh Assembly.
Mr Hamilton - who is backing MEP Mr Arnott for the role - has said Mr Woolfe should not be able to run for the party's leadership if his nomination was submitted late.
Meanwhile, Mr Carswell, the party's only MP, has said he will not be endorsing any of the candidates but suggested they needed to consider what UKIP's purpose was after the EU referendum, how it should go about increasing its elected representation and how the party's leadership style needed to change.
UKIP leadership: Final list of candidates to be named - BBC News
UKIP leadership: The main contenders to succeed Nigel Farage
Lisa Duffy - standing
Although not a well-known figure in Westminster circles, Lisa Duffy has won the backing of key modernising figures in the party such as Suzanne Evans and Patrick O'Flynn.
Ms Duffy is a town and district (Huntingdonshire) councillor in Cambridgeshire, and a former mayor of the town of Ramsey, and as campaigns director played a key role in fighting by-elections.
At her campaign launch, Ms Duffy, 48, a mother of six and former retail manager, pitched herself as the "grassroots candidate" who would "walk the People's Army into Westminster".
Chief of staff to MEP Mr O'Flynn, she joined UKIP in 2004 and stood unsuccessfully against Labour's Hazel Blears in Salford in the 2005 general election.
She said the party needed to widen its appeal to win seats in 2020 but also continue to tackle "taboo subjects" such as the impact of immigration on British communities and the need for a positive vision for "modernised British Islam". She has called for a "total ban" on Muslim state schools.
Jonathan Arnott - standing
The North East UKIP MEP has thrown his hat into the leadership race, saying he wants to be a "strong and credible voice for the North East".
The 35-year-old was elected to the European Parliament in 2014 and is a former general secretary of the party.
Announcing his candidacy, he said he wanted to appeal to the millions of voters who did not support UKIP in last year's general election, but backed Brexit in the EU referendum.
He warned that the elevation of Theresa May to the premiership meant that "negotiations for the UK's withdrawal from the EU will be in the hands of those who do not actually want us to leave".
Bill Etheridge - standing
Mr Etheridge has called on UKIP to stay true to its values An MEP for the West Midlands, and Dudley councillor, he has said UKIP should stick by its "core principles and values" and not be "distracted by negative influences".
The 46-year old was a Conservative activist before joining in 2011.
He has called for major reform of the penal system, including restricting prisoners' privileges, banning visits during the first six months of a sentence and an automatic 10-year increase in sentence for anyone attacking a prison officer.
He also backs far-reaching changes to the tax system, including a 50% cut in alcohol and tobacco duty, replacing VAT with a local sales tax and gradually merging national insurance and income taxes.
Elizabeth Jones - standing
Potentially the surprise package in the field, Elizabeth Jones is one of the party's leading figures in London, although she has less of a national profile.
Deputy chair of the party's Lambeth branch, she stood unsuccessfully in May's London Assembly elections and came fifth in last month's Tooting by-election.
The family law solicitor is also a member of the party's national executive committee.
She has said she will bring a fresh approach and different style of leadership, urging a focus on trade and education in the wake of the vote to leave the EU.
Steven Woolfe - wants to stand, but missed nominations deadline
The 48-year-old barrister was regarded as the favourite to succeed Mr Farage but his candidacy has come under increased scrutiny after a number of apparent mishaps.
He missed the deadline for submitting his nomination, blaming "technical issues" in sending the paperwork online.
He was forced to dismiss as "absolute nonsense" suggestions his leadership bid could be derailed amid reports that his UKIP membership lapsed between the end of 2014 and early 2016 - party rules require candidates to have been members for two years.
And he has admitted that he failed to disclose a drink-driving ban from 2002 when seeking election as a police and crime commissioner a decade later.
Mr Woolfe, who holds the high-profile immigration and financial affairs brief, joined UKIP in 2010 and was elected as MEP for North West England in 2014.
Announcing his leadership bid, he promised to tackle social mobility by backing 50 new grammar schools in deprived areas of the UK and to chase Labour's vote "ruthlessly".
Born in Manchester, with African-American, Jewish and Irish Catholic roots, he is also head of the party's Black and Minority Ethnic (BME) association.
After being called to the bar, he practised in criminal law before moving into commercial practice, advising hedge funds and other City businesses on regulatory issues.
Diane James
The 56-year-old former businesswomen and healthcare professional is one of the party's most fluent and reliable media performers.
It is believed she has thrown her hat into the ring although this is yet to be confirmed, with the final list of candidates scheduled to be announced on Tuesday.
She came within fewer than 2,000 votes of becoming UKIP's first elected MP when she narrowly lost the 2013 Eastleigh by-election to the Lib Dems, impressing journalists with her performance - which relegated the Conservatives into third place.
She went on to be elected to the European Parliament as MEP for South East England in 2014, although she decided against standing in the 2015 general election for personal reasons.
As deputy chair and home affairs and justice spokeswoman, she has maintained a high profile, although it remains to be seen whether she has the breadth of appeal to prevail in what is likely to be a fierce contest.
Philip Broughton
He stood as a parliamentary candidate in Hartlepool at the 2015 general election, narrowly losing to incumbent Labour MP Iain Wright, who retained the seat with a majority of 3,024 - down from 5,509.
He says UKIP has "won the arguments" on Brexit and now needs to "broaden our message" - on the economy, NHS, defence, law and order, foreign policy and foreign aid.
Mr Broughton, who describes himself as a "young, passionate, northern, working class guy", is a semi-professional wrestler, and works as a supervisor in a local Tesco.
He used to be a Conservative councillor - between 2007 and 2011 - before he defected to UKIP.
Ruled out
Those not standing for the leadership include:
London Assembly member Peter Whittle, who had said he was thinking about running but later chose to support Steven Woolfe.
Suzanne Evans, a Conservative councillor who defected to UKIP in 2013, who is currently suspended from the party until September following an internal dispute.
Clacton MP Douglas Carswell, the party's best-known figure aside from Mr Farage, ruled himself out of the contest.
Paul Nuttall - described by Mr Farage as the best deputy leader of any political party in the UK - also ruled himself out, saying he had achieved his objective of getting the UK out of the EU.
Patrick O'Flynn, a former Press Association and Daily Express journalist, has given his support to Lisa Duffy.
UKIP leadership: The main contenders to succeed Nigel Farage - BBC News
The party has been carrying out "one final round" of vetting to determine who is eligible to stand in the race.
UKIP donor Arron Banks suggested Steven Woolfe will not be on the ballot.
Mr Woolfe's candidacy is in officials' hands after he missed the deadline and revealed he did not disclose a drink-driving ban when he stood to be Greater Manchester's Police and Crime Commissioner in 2012.
Also in the race are MEPs Jonathan Arnott and Bill Etheridge, while Huntingdonshire councillor Lisa Duffy and activists Phillip Broughton and Elizabeth Jones have also put their names forward.
There is continued uncertainty about whether prominent MEP Diane James has applied.
The contest has been sparked by Mr Farage's decision to stand down following the UK's vote to leave the EU, saying his "political ambition has been achieved".
UKIP leadership: Final list of candidates to be named
BBC News
3 August 2016
Nigel Farage announced he was stepping down after the UK voted to leave the EU
The final list of candidates in the running to succeed Nigel Farage as UKIP leader will be announced later.
The party has been carrying out "one final round" of vetting to determine who is eligible to stand in the race.
UKIP donor Arron Banks suggested Steven Woolfe will not be on the ballot.
Mr Woolfe's candidacy is in officials' hands after he missed the deadline and revealed he did not disclose a drink-driving ban when he stood to be Greater Manchester's Police and Crime Commissioner in 2012.
Also in the race are MEPs Jonathan Arnott and Bill Etheridge, while Huntingdonshire councillor Lisa Duffy and activists Phillip Broughton and Elizabeth Jones have also put their names forward.
There is continued uncertainty about whether prominent MEP Diane James has applied.
The contest has been sparked by Mr Farage's decision to stand down following the UK's vote to leave the EU, saying his "political ambition has been achieved".
Succession issue
Steven Woolfe
The vetting panel had been expected to announce on Tuesday which candidates were eligible to stand under the party's rules after nominations closed on Sunday.
But after the panel met it issued a statement saying the final list of leadership candidates would be revealed on Wednesday at midday.
"An NEC-led panel has today met to determine the eligibility of those candidates that submitted their nomination papers to stand for party leader.
"That panel has now met and with one final round of checking to go the party aims to announce the full list of candidates at midday tomorrow, Wednesday 3 August," a party spokesman said.
To qualify, candidates are expected to have been party members for two years and to have been nominated by 50 members of the party.
Leadership contest timetable
31 July: Nominations closed
3 August: Final list of candidates announced
1 September: Ballot papers issued to party members
15 September: Votes counted and the winner of the contest announced
Steve Crowther, who left his job as UKIP chairman earlier this week, would not be drawn on whether Mr Woolfe - seen as the frontrunner in the race - should be excluded from the contest for submitting his papers 15 minutes late - a delay that the MEP has blamed on computer problems.
But asked about the issue of the drink-driving conviction - which Mr Woolfe incurred in 2002 but which he did not reveal when he stood to become a police and crime commissioner in 2012 in a possible breach of electoral law - he told the BBC that this was "arguably more serious".
'Coup claim'
Mr Woolfe got a £350 fine and nine-month ban for being drunk in charge of a scooter but insisted his life had moved on by 2012 and the conviction was spent by then.
Amid reports of a power struggle within the party, UKIP NEC member Victoria Ayling tweeted on Tuesday evening to urge her fellow panel members not to "block" Mr Woolfe.
UKIP donor Arron Banks later tweeted: "Tonight's decision to exclude Woolfe is the final straw" and he claimed it represented a "coup" by UKIP MP Douglas Carswell and Neil Hamilton, UKIP's leader in the Welsh Assembly.
Mr Hamilton - who is backing MEP Mr Arnott for the role - has said Mr Woolfe should not be able to run for the party's leadership if his nomination was submitted late.
Meanwhile, Mr Carswell, the party's only MP, has said he will not be endorsing any of the candidates but suggested they needed to consider what UKIP's purpose was after the EU referendum, how it should go about increasing its elected representation and how the party's leadership style needed to change.
UKIP leadership: Final list of candidates to be named - BBC News
UKIP leadership: The main contenders to succeed Nigel Farage
Lisa Duffy - standing
Although not a well-known figure in Westminster circles, Lisa Duffy has won the backing of key modernising figures in the party such as Suzanne Evans and Patrick O'Flynn.
Ms Duffy is a town and district (Huntingdonshire) councillor in Cambridgeshire, and a former mayor of the town of Ramsey, and as campaigns director played a key role in fighting by-elections.
At her campaign launch, Ms Duffy, 48, a mother of six and former retail manager, pitched herself as the "grassroots candidate" who would "walk the People's Army into Westminster".
Chief of staff to MEP Mr O'Flynn, she joined UKIP in 2004 and stood unsuccessfully against Labour's Hazel Blears in Salford in the 2005 general election.
She said the party needed to widen its appeal to win seats in 2020 but also continue to tackle "taboo subjects" such as the impact of immigration on British communities and the need for a positive vision for "modernised British Islam". She has called for a "total ban" on Muslim state schools.
Jonathan Arnott - standing
The North East UKIP MEP has thrown his hat into the leadership race, saying he wants to be a "strong and credible voice for the North East".
The 35-year-old was elected to the European Parliament in 2014 and is a former general secretary of the party.
Announcing his candidacy, he said he wanted to appeal to the millions of voters who did not support UKIP in last year's general election, but backed Brexit in the EU referendum.
He warned that the elevation of Theresa May to the premiership meant that "negotiations for the UK's withdrawal from the EU will be in the hands of those who do not actually want us to leave".
Bill Etheridge - standing
Mr Etheridge has called on UKIP to stay true to its values An MEP for the West Midlands, and Dudley councillor, he has said UKIP should stick by its "core principles and values" and not be "distracted by negative influences".
The 46-year old was a Conservative activist before joining in 2011.
He has called for major reform of the penal system, including restricting prisoners' privileges, banning visits during the first six months of a sentence and an automatic 10-year increase in sentence for anyone attacking a prison officer.
He also backs far-reaching changes to the tax system, including a 50% cut in alcohol and tobacco duty, replacing VAT with a local sales tax and gradually merging national insurance and income taxes.
Elizabeth Jones - standing
Potentially the surprise package in the field, Elizabeth Jones is one of the party's leading figures in London, although she has less of a national profile.
Deputy chair of the party's Lambeth branch, she stood unsuccessfully in May's London Assembly elections and came fifth in last month's Tooting by-election.
The family law solicitor is also a member of the party's national executive committee.
She has said she will bring a fresh approach and different style of leadership, urging a focus on trade and education in the wake of the vote to leave the EU.
Steven Woolfe - wants to stand, but missed nominations deadline
The 48-year-old barrister was regarded as the favourite to succeed Mr Farage but his candidacy has come under increased scrutiny after a number of apparent mishaps.
He missed the deadline for submitting his nomination, blaming "technical issues" in sending the paperwork online.
He was forced to dismiss as "absolute nonsense" suggestions his leadership bid could be derailed amid reports that his UKIP membership lapsed between the end of 2014 and early 2016 - party rules require candidates to have been members for two years.
And he has admitted that he failed to disclose a drink-driving ban from 2002 when seeking election as a police and crime commissioner a decade later.
Mr Woolfe, who holds the high-profile immigration and financial affairs brief, joined UKIP in 2010 and was elected as MEP for North West England in 2014.
Announcing his leadership bid, he promised to tackle social mobility by backing 50 new grammar schools in deprived areas of the UK and to chase Labour's vote "ruthlessly".
Born in Manchester, with African-American, Jewish and Irish Catholic roots, he is also head of the party's Black and Minority Ethnic (BME) association.
After being called to the bar, he practised in criminal law before moving into commercial practice, advising hedge funds and other City businesses on regulatory issues.
Diane James
The 56-year-old former businesswomen and healthcare professional is one of the party's most fluent and reliable media performers.
It is believed she has thrown her hat into the ring although this is yet to be confirmed, with the final list of candidates scheduled to be announced on Tuesday.
She came within fewer than 2,000 votes of becoming UKIP's first elected MP when she narrowly lost the 2013 Eastleigh by-election to the Lib Dems, impressing journalists with her performance - which relegated the Conservatives into third place.
She went on to be elected to the European Parliament as MEP for South East England in 2014, although she decided against standing in the 2015 general election for personal reasons.
As deputy chair and home affairs and justice spokeswoman, she has maintained a high profile, although it remains to be seen whether she has the breadth of appeal to prevail in what is likely to be a fierce contest.
Philip Broughton
He stood as a parliamentary candidate in Hartlepool at the 2015 general election, narrowly losing to incumbent Labour MP Iain Wright, who retained the seat with a majority of 3,024 - down from 5,509.
He says UKIP has "won the arguments" on Brexit and now needs to "broaden our message" - on the economy, NHS, defence, law and order, foreign policy and foreign aid.
Mr Broughton, who describes himself as a "young, passionate, northern, working class guy", is a semi-professional wrestler, and works as a supervisor in a local Tesco.
He used to be a Conservative councillor - between 2007 and 2011 - before he defected to UKIP.
Ruled out
Those not standing for the leadership include:
London Assembly member Peter Whittle, who had said he was thinking about running but later chose to support Steven Woolfe.
Suzanne Evans, a Conservative councillor who defected to UKIP in 2013, who is currently suspended from the party until September following an internal dispute.
Clacton MP Douglas Carswell, the party's best-known figure aside from Mr Farage, ruled himself out of the contest.
Paul Nuttall - described by Mr Farage as the best deputy leader of any political party in the UK - also ruled himself out, saying he had achieved his objective of getting the UK out of the EU.
Patrick O'Flynn, a former Press Association and Daily Express journalist, has given his support to Lisa Duffy.
UKIP leadership: The main contenders to succeed Nigel Farage - BBC News
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