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'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.

In evidence to the inquiry last month, 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".