Jack Straw will return to the Chilcot Inquiry today to face questions about why he rejected advice that the Iraq war was illegal.
Mr Straw, who was foreign secretary at the time of the 2003 invasion, said last month that he only "very reluctantly" came round to backing the conflict.
But declassified documents since released by the inquiry show that he actively supported the view that the war would be lawful without further United Nations backing.
Mr Straw dismissed the advice of his senior legal adviser at the Foreign Office, Sir Michael Wood, that attacking Iraq without a UN Security Council resolution specifically authorising military action would be a "crime of aggression".
And he wrote to then-attorney general Lord Goldsmith six weeks before the March 2003 invasion urging him to change his initial view that another resolution was needed.
The inquiry has heard that Sir Michael took issue with Mr Straw in January 2003 over his assertion in a meeting with US vice president Dick Cheney that Britain would still be "OK" if it failed to get a second resolution.
He wrote to the then-foreign secretary in a memo: "To use force without Security Council authority would amount to a crime of aggression."
Mr Straw replied: "I note your advice but I do not accept it."
Sir Michael told the inquiry: "He took the view that I was being very dogmatic and that international law was pretty vague and that he wasn't used to people taking such a firm position.
"When he had been at the Home Office, he had often been advised things were unlawful but he had gone ahead anyway and won in the courts."
Sir Michael's deputy, Elizabeth Wilmshurst, who resigned in protest at the war, said the Foreign Office lawyers were united in their belief of the need for a second resolution.
Lord Goldsmith himself advised former prime minister Tony Blair in January 2003 that the war would be unlawful without another Security Council resolution.
But he changed his mind after consulting with senior US administration lawyers, Britain's ambassador to the UN, Sir Jeremy Greenstock, and Mr Straw.
Mr Straw, who is now the Justice Secretary, wrote to the attorney general on February 6 arguing for a legal interpretation on military action "which coincides with our firm policy intention".
On March 7 Lord Goldsmith presented Mr Blair with a 13-page legal opinion in which he said a "reasonable case" could be made for attacking Iraq without further Security Council support.
Ten days later he stated without reservation in a short written statement to Parliament that an existing UN resolution provided the necessary authority for the conflict.
In evidence to the inquiry last month, Mr Straw said he presented Mr Blair with an alternative plan on the eve of the crucial Commons vote on war which did not involve committing British troops alongside the Americans.
He said his support for the invasion had been "critical" if UK forces were to be involved as Mr Blair could not have carried the Government and Parliament without him.
While he had not actually considered resigning, he nevertheless said he had weighed his responsibilities in relation to the policy on Iraq "very heavily".
Throughout his evidence Mr Straw repeatedly appeared to suggest that his views were at odds with Mr Blair, saying that he owed the Prime Minister his loyalty but they were "two different people".
Mr Straw said a policy of simple "regime change" in Iraq - as the Americans were advocating - would have been "improper and self-evidently unlawful" and he would not have gone along with it.
The inquiry will also hear evidence today from General Sir John McColl, the senior British military representative in Iraq in 2004.Reuse content