Straw 'very reluctantly' backed invasion of Iraq

He had presented former Prime Minister Tony 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 had been "critical" if UK forces were to be involved as Mr Blair could not have carried the Government and Parliament without him.

But while he had not actually considered resigning, he nevertheless said he had weighed his responsibilities "very heavily".

Throughout his evidence yesterday he repeatedly appeared to suggest his views were at odds with Mr Blair, saying that, while he owed the Prime Minister his loyalty, they were "two different people".

A policy which simply sought to overthrow Saddam Hussein – as the Americans were advocating and some witnesses have suggested Mr Blair wanted – would have been "improper and self-evidently unlawful" and he would not have gone along with it.

Throughout the build-up to the invasion, he said he had always believed it was possible to resolve the crisis over Saddam's supposed weapons of mass destruction without the need for military action.

And he bitterly attacked both French President Jacques Chirac and chief United Nations weapons inspector Hans Blix for scuppering diplomatic efforts to find a peaceful resolution to the crisis.

Pressed on whether he had actually considered quitting the Cabinet rather than support military action, he said: "Did I ever think, I'm going to resign over this? No, I didn't.

"We all have our bottom lines. Did I understand the nature of the responsibilities on me? Yes, I did, for sure.

"But in the event, I came to the decisions that I came to. I did so very reluctantly, but on what I judged to be the best evidence available at the time."

He confirmed that he had drawn up contingency plans in case the Government lost the crucial Commons vote on March 18, on the eve of the invasion, making British participation impossible.

Mr Straw said that, from the time of the Prime Minister's meeting with George Bush at the US President's Texas ranch in Crawford in April 2002, he had made clear that Britain could not join a US-led military invasion without a proper UN mandate.

The inquiry continues.