Clare Short arranged a time to make her resignation speech in the House of Commons in the run-up to the Iraq war, she has told an inquiry into the conflict.
The MP for Birmingham Ladywood revealed for the first time how close she had come to quitting the cabinet and potentially changing history.
She was planning to leave her job as Secretary of State for International Development, severely weakening Tony Blair’s position as he tried to win the support of MPs for the invasion of Iraq.
But the former Prime Minister begged her to stay - because he realised how damaging it would be if both Ms Short and Robin Cook, the former Foreign Secretary, stood down.
In the end, only Mr Cook left the Government while Ms Short remained in the Cabinet as the conflict begun.
Her continued support for Mr Blair helped the Prime Minister win a crucial Commons vote on the war in March 2003, when 139 Labour backbenchers voted against the Government.
Ms Short told the official inquiry into the Iraq War that she stayed after Mr Blair promised the United Nations would oversee the reconstruction of Iraq, and that US President George Bush would push for a Palestinian state. But those pledges were broken, she said.
She said: “I had booked my place to make my resignation statement with the speaker and the Prime Minister.”
But instead, she gave in to Tony Blair’s pleas to stay, believing she could do more good in the Cabinet than outside.
“I thought if we get the Palestinian state and a UN lead on reconstruction, that will be a heck of a lot better than what might otherwise happen.”
Ms Short eventually quit the Cabinet in May 2003, but she said she regretted not resigning sooner. She told the inquiry: “If I knew then what I know now, I would have.”
The Birmingham MP said she asked for extra money to prepare for aftermath Iraq but received no reply from the Treasury.
Letters declassified ahead of the hearing show Ms Short wrote to Mr Blair on March 5, 2003 warning that it would be “impossible” to take a “leading role in humanitarian delivery” without more cash.
She said Gordon Brown, the then Chancellor, was being “marginalised” in the weeks before the invasion.
“Brown was pushed out and marginalised at the time and having cups of coffee with me and saying ‘Tony Blair is obsessed with his legacy and he thinks he can have a quick war and then a reshuffle etc’,” she said.
And Ms Short was damning about Mr Blair’s failure to ask Washington to delay the invasion despite warnings that the military and aid officials were not ready.
She said: “I think he was so frantic to be with America that all that was thrown away.
“If he had done that, his place in history and the UK’s role in the world would have been so much more honourable.
“Britain needs to think about this, the special relationship. What do we mean by it?
“Do we mean we have an independent relationship and we say what we think, or do we mean we just abjectly go wherever America goes and that puts us in the big league? That’s a tragedy.”