Tony Blair's cabinet was "misled" into thinking the war with Iraq was legal, ex-International Development Secretary Clare Short told the UK's inquiry.
Attorney General Lord Goldsmith had been "leaned on" to change his advice before the invasion, she said.
Ms Short said Mr Blair "and his mates" decided war was necessary, and "everything was done on a wing and a prayer".
She quit the cabinet two months after the March 2003 invasion, in protest at planning for the war's aftermath.
In her evidence to the Iraq inquiry, during which she was highly critical of former Prime Minister Tony Blair, she said the cabinet had not been a "decision-making body" and called Parliament a "rubber stamp".
Ms Short, who was given a round of applause after her three-hour appearance, added that she had been "conned" into staying on as a minister until May 2003, despite her misgivings about the war.
The attorney general provisionally advised Mr Blair in January that year that it would be unlawful to invade Iraq without a further United Nations Security Council resolution.
But he changed his mind a month later after being persuaded to talk to senior US government lawyers and Britain's ambassador to the UN, Sir Jeremy Greenstock.
A definitive statement circulated at cabinet on 17 March 2003, three days before the war began.
Ms Short said there was no suggestion given that he had had any legal doubts, and said that any discussion of the legal advice was halted at that pre-war cabinet meeting.
She had been "shocked" that the attorney general's advice was so late but was "jeered at" to be quiet by other ministers when she asked why.
Ms Short said that, when she repeated the question to Lord Goldsmith, he had replied: "Oh, it takes me a long time to make my mind up."
In light of the attorney general's "doubts and his changes of opinion" that have since emerged, Ms Short said: "I think for the attorney general to come and say there's unequivocal legal authority to go war was misleading."
She said: "I think he misled the cabinet. He certainly misled me, but people let it through."
Ms Short said that, after the failure to secure a second UN resolution, the government had put out "untrue" claims that France had vetoed it.
But she added that she "believed them at the time. You don't want to disbelieve your prime minister in the run-up to war and you want to believe the leader of your party. You want to be loyal".
Asked why she did not resign earlier, like her cabinet colleague Robin Cook, Ms Short said: "I was conned."
But Ms Short told the inquiry Mr Blair's evidence was "historically inaccurate", adding: "There was no evidence of any kind of an escalation of threats."
She also said: "We could have gone more slowly and carefully and not have had a totally destabilised and angry Iraq.
"The American people were misled to suggest that al-Qaeda had links to Saddam Hussein.
"Everybody knows that is untrue - that he had absolutely no links, no sympathy, al-Qaeda were nowhere near Iraq until after the invasion and the disorder that came from that."