In a lengthy submission to the official Iraq Inquiry, the former foreign secretary described his decision to back military action in March 2003 as ''the most difficult I have ever faced in my life''.
But Mr Straw, who is now the Justice Secretary, said he had never ''backed away'' from the choice that he made then and fully accepted the responsibilities that flowed from it.
He said the question of whether to back military action in support of the US had posed a ''moral as well as political dilemma'' which was ''profoundly difficult''.
He acknowledged that if he had refused to support it, Mr Blair would have been unable to carry the Government or Parliament.
''I was fully aware that my support for military action was critical,'' he said. ''If I had refused that, the UK's participation in the military action would not in practice have been possible.
''There almost certainly would have been no majority in either Cabinet or in the Commons.''
Mr Straw acknowledged that the failure to find weapons of mass destruction - the justification for the invasion - had "undermined trust" but insisted that the decision to go to war was taken on the basis of the best available evidence.
"I made my choice. I have never backed away from it, and I do not intend to do so, and fully accept the responsibilities which flow from that," he said.
"I believed at the time, and still believe, that we made the best judgments we could have done in the circumstances. We did so assiduously, and on the best evidence we had available at the time."
He said that while intelligence played a "significant part" in the assessment of the threat posed by Saddam Hussein, it was not the only factor.
"My starting point on the assessment of the threat was what was publicly known about Iraq's WMD programmes, and its behaviour going back more than a dozen years," he said.
"It was that judgment - not intelligence - which lay at the heart of the UK Government's strategy for disarming Iraq, by diplomacy backed by the potential use of force."
He underlined the importance of "staying close" to the US in British decision-making at the time.
"There was also the enduring spectre of Suez over British foreign policy, which led to an all-pervasive view that the UK should so far as possible seek to 'stay close' to the United States," he said.
"I shared with the Prime Minister the view that the best approach for the UK was indeed to 'stay close' to the US administration and seek to persuade them that any action against Iraq had to be through the United Nations."
Mr Straw told the inquiry it was never British policy to achieve "regime change" in Iraq.
He pointed out that overthrowing Saddam had been part of official US policy since an Act of Congress signed by former president Bill Clinton in 1998.
But it was only after the September 11 2001 attacks that the Americans began to decide that they should do something about it, he said.
Mr Straw said: "We did not share the policy of regime change as a purpose of our foreign policy with the United States. It was not our policy in 2002, it was not our policy in 2003 and there would have been no legal base for it ever to be our policy."