UKIP has been plunged into turmoil after Diane James unexpectedly quit as leader after 18 days in the job.
Ms James cited professional and personal reasons for her decision.
Ms James, who succeeded Nigel Farage on 16 September, said she did not have enough authority in the party and would not be "formalising" her nomination.
She had not appointed a deputy and UKIP officials were unable to say who was now the leader of the party, although Mr Farage says he technically still is.
However, Mr Farage told the BBC that he would not be contesting any future leadership contest.
UKIP in leadership turmoil after Diane James resignation
BBC News
5 October 2016
UKIP has been plunged into turmoil after Diane James unexpectedly quit as leader after 18 days in the job.
Ms James cited professional and personal reasons for her decision.
Ms James, who succeeded Nigel Farage on 16 September, said she did not have enough authority in the party and would not be "formalising" her nomination.
She had not appointed a deputy and UKIP officials were unable to say who was now the leader of the party, although Mr Farage says he technically still is.
However, Mr Farage told the BBC that he would not be contesting any future leadership contest.
Speaking to BBC Radio 4's Today programme, UKIP chairman Paul Oakden said he would check with the Electoral Commission to see who the leader was and admitted it could technically be Mr Farage.
Asked whether Mr Farage would return to lead the party again, Mr Oakden said it was "very unlikely" but added: "I wouldn't say anything is impossible."
Mr Oakden previously said he would look to hold an emergency meeting of the party's national executive committee to confirm the process for electing Ms James's replacement.
"Whilst the decision is unfortunate, it is one that Diane is entitled to make. We thank her for all her work as leader, and as a hard-working MEP, a role she will continue with her customary vigour."
Ms James said in her resignation statement sent to The Times newspaper: "It has become clear that I do not have sufficient authority, nor the full support of all my MEP colleagues and party officers to implement changes I believe necessary and upon which I based my campaign.
"For personal and professional reasons therefore, I will not take the election process further."
The former Conservative councillor, who defected to UKIP in 2011, said she would "continue to concentrate fully" on her activities and responsibilities as an MEP, adding it was her "final media statement on the issue".
Her decision is also thought to be partly due to a family illness.
Ms James, a former businesswoman and healthcare professional, defeated four other contenders - Lisa Duffy, Bill Etheridge, Liz Jones and Phillip Broughton - in the leadership battle.
She won with 8,451 out of the 17,970 votes cast, ahead of Ms Duffy, who received 4,591.
One-time frontrunner Steven Woolfe was barred from standing after submitting his nomination papers too late, while Suzanne Evans was unable to take part as she was suspended at the time.
Speaking to Today, Ms Duffy said she was "not overly surprised" at the news Ms James had quit, saying she did not believe she had "particularly wanted to do (the job) or had the passion" for it.
"She entered the leadership contest at the last minute, didn't engage in the process and over the last 18 days I'm afraid we haven't had any leadership from her," Ms Duffy said, adding that she was "disappointed" for UKIP members.
Councillor Lisa Duffy said she would consider whether to run for the leadership again
Ms Duffy said she would not rule herself out from standing again for the leadership.
She added: "I've said to the party chairman, 'I am here right now, anything I can do to support our party and our hard-working members just pick up the phone and ask, I'm available'."
On UKIP's future, Ms Duffy insisted the party was "more relevant than we've ever been", and said a strong leader is needed to set the party's direction.
Diane James addresses the UKIP party conference after being elected its new leader.
Ms James, in her acceptance speech at the party's conference in Bournemouth, had said UKIP would "confound our critics", "outwit our opponents" and "build on our election success" and more.
But the race to replace Mr Farage had exposed bitter divisions within UKIP, with Ms James encountering criticism from some in the party after declining to take part in hustings debates around the country with her rivals.
Her resignation prompted speculation Mr Farage - UKIP leader from 2006-2009 and 2010-2016 - might return to lead the party for a third time.
However, he told the Press Association: "I'm not coming back, I'm retired."
Nigel Farage has ruled out a leadership return
The face of Euroscepticism in the UK for nearly two decades, Mr Farage helped turn UKIP from a fringe party into the third biggest in UK politics - in terms of votes at the 2015 general election.
Announcing his decision to stand down, he said his "political ambition has been achieved".
Responding to Ms James's resignation, UKIP's only MP, Douglas Carswell, said: "I'm sad and surprised. I just hope she's alright".
Meanwhile, a number of senior UKIP members in the East of England have told the BBC of having "nagging concerns" about the "radio silence" from Ms James's camp since her election as leader.
They suggested that unless UKIP's national executive committee changes party rules, Lisa Duffy - who was backed by many anti-Farage supporters - "made sense" as an interim leader.
Analysis
Norman Smith, BBC assistant political editor
Even before Diane James' shock resignation, UKIP were a party teetering on the cusp of chaos, riven by feuds and factionalism.
Nigel Farage's decision to quit in the summer stripped UKIP of his dominant, driving personality, which held a quarrelling party together and gave it a sense of identity and momentum.
The subsequent brutal leadership contest exposed the personal and political fault lines with the party descending into near civil war.
On top of that, victory in the Brexit referendum had appeared to deprive the party of its very reason for existing.
Inevitably there will be speculation about a possible return for Mr Farage, though last night he insisted he had retired and would not be coming back.
Now with a slow haemorrhage of support returning to the Tories, there will be questions as to whether UKIP faces the fate of so many minor parties squeezed to the out margins of British politics.
UKIP in leadership turmoil after Diane James resignation - BBC News
Ms James cited professional and personal reasons for her decision.
Ms James, who succeeded Nigel Farage on 16 September, said she did not have enough authority in the party and would not be "formalising" her nomination.
She had not appointed a deputy and UKIP officials were unable to say who was now the leader of the party, although Mr Farage says he technically still is.
However, Mr Farage told the BBC that he would not be contesting any future leadership contest.
UKIP in leadership turmoil after Diane James resignation
BBC News
5 October 2016
UKIP has been plunged into turmoil after Diane James unexpectedly quit as leader after 18 days in the job.
Ms James cited professional and personal reasons for her decision.
Ms James, who succeeded Nigel Farage on 16 September, said she did not have enough authority in the party and would not be "formalising" her nomination.
She had not appointed a deputy and UKIP officials were unable to say who was now the leader of the party, although Mr Farage says he technically still is.
However, Mr Farage told the BBC that he would not be contesting any future leadership contest.
Speaking to BBC Radio 4's Today programme, UKIP chairman Paul Oakden said he would check with the Electoral Commission to see who the leader was and admitted it could technically be Mr Farage.
Asked whether Mr Farage would return to lead the party again, Mr Oakden said it was "very unlikely" but added: "I wouldn't say anything is impossible."
Mr Oakden previously said he would look to hold an emergency meeting of the party's national executive committee to confirm the process for electing Ms James's replacement.
"Whilst the decision is unfortunate, it is one that Diane is entitled to make. We thank her for all her work as leader, and as a hard-working MEP, a role she will continue with her customary vigour."
Ms James said in her resignation statement sent to The Times newspaper: "It has become clear that I do not have sufficient authority, nor the full support of all my MEP colleagues and party officers to implement changes I believe necessary and upon which I based my campaign.
"For personal and professional reasons therefore, I will not take the election process further."
The former Conservative councillor, who defected to UKIP in 2011, said she would "continue to concentrate fully" on her activities and responsibilities as an MEP, adding it was her "final media statement on the issue".
Her decision is also thought to be partly due to a family illness.
Ms James, a former businesswoman and healthcare professional, defeated four other contenders - Lisa Duffy, Bill Etheridge, Liz Jones and Phillip Broughton - in the leadership battle.
She won with 8,451 out of the 17,970 votes cast, ahead of Ms Duffy, who received 4,591.
One-time frontrunner Steven Woolfe was barred from standing after submitting his nomination papers too late, while Suzanne Evans was unable to take part as she was suspended at the time.
Speaking to Today, Ms Duffy said she was "not overly surprised" at the news Ms James had quit, saying she did not believe she had "particularly wanted to do (the job) or had the passion" for it.
"She entered the leadership contest at the last minute, didn't engage in the process and over the last 18 days I'm afraid we haven't had any leadership from her," Ms Duffy said, adding that she was "disappointed" for UKIP members.
Councillor Lisa Duffy said she would consider whether to run for the leadership again
Ms Duffy said she would not rule herself out from standing again for the leadership.
She added: "I've said to the party chairman, 'I am here right now, anything I can do to support our party and our hard-working members just pick up the phone and ask, I'm available'."
On UKIP's future, Ms Duffy insisted the party was "more relevant than we've ever been", and said a strong leader is needed to set the party's direction.
Diane James addresses the UKIP party conference after being elected its new leader.
Ms James, in her acceptance speech at the party's conference in Bournemouth, had said UKIP would "confound our critics", "outwit our opponents" and "build on our election success" and more.
But the race to replace Mr Farage had exposed bitter divisions within UKIP, with Ms James encountering criticism from some in the party after declining to take part in hustings debates around the country with her rivals.
Her resignation prompted speculation Mr Farage - UKIP leader from 2006-2009 and 2010-2016 - might return to lead the party for a third time.
However, he told the Press Association: "I'm not coming back, I'm retired."
Nigel Farage has ruled out a leadership return
The face of Euroscepticism in the UK for nearly two decades, Mr Farage helped turn UKIP from a fringe party into the third biggest in UK politics - in terms of votes at the 2015 general election.
Announcing his decision to stand down, he said his "political ambition has been achieved".
Responding to Ms James's resignation, UKIP's only MP, Douglas Carswell, said: "I'm sad and surprised. I just hope she's alright".
Meanwhile, a number of senior UKIP members in the East of England have told the BBC of having "nagging concerns" about the "radio silence" from Ms James's camp since her election as leader.
They suggested that unless UKIP's national executive committee changes party rules, Lisa Duffy - who was backed by many anti-Farage supporters - "made sense" as an interim leader.
Analysis
Norman Smith, BBC assistant political editor
Even before Diane James' shock resignation, UKIP were a party teetering on the cusp of chaos, riven by feuds and factionalism.
Nigel Farage's decision to quit in the summer stripped UKIP of his dominant, driving personality, which held a quarrelling party together and gave it a sense of identity and momentum.
The subsequent brutal leadership contest exposed the personal and political fault lines with the party descending into near civil war.
On top of that, victory in the Brexit referendum had appeared to deprive the party of its very reason for existing.
Inevitably there will be speculation about a possible return for Mr Farage, though last night he insisted he had retired and would not be coming back.
Now with a slow haemorrhage of support returning to the Tories, there will be questions as to whether UKIP faces the fate of so many minor parties squeezed to the out margins of British politics.
UKIP in leadership turmoil after Diane James resignation - BBC News
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