Michael Wood, who was the chief legal adviser to the Foreign Office at the time of the 2003 invasion, told the inquiry in London he thought the use of force against Iraq was "contrary to international law."
"In my opinion, that use of force had not been authorized by the [United Nations] Security Council, and had no other legal basis in international law," he said in a statement before the inquiry on Tuesday afternoon.
Wood also told the inquiry that former foreign secretary Jack Straw disagreed with his assessment and argued instead that international law remained vague and offered some leeway. Wood said that when he disagreed, Straw told him he was being "dogmatic."
Wood's former deputy, Elizabeth Wilmshurst, resigned in protest at Britain's decision to send troops to Iraq.
Staff member resigned over invasion
Wilmshurst is scheduled to speak later on Tuesday, but in advance of the hearing released a statement saying the UN charter condones collective security and not unilateral military action.
"Acting contrary to the charter, as I perceived the government to be doing, would have the consequence of damaging the United Kingdom's reputation as a state committed to the rule of law in international relations and to the United Nations."
The panel hearing testimony at the inquiry won't lay blame or establish criminal or civil liability, but will give reprimands if warranted and make recommendations.
Former prime minister Tony Blair himself is expected to appear in late January or early February.
The British military formally ended six years of combat operations in Iraq on April 30, 2009, handing over control of its main base in Basra to a U.S. brigade.
A total of 179 British soldiers lost their lives during the U.K. involvement in Iraq.