Clare Short, the former international development secretary, today accused Tony Blair of lying to her and misleading parliament in the build-up to the Iraq invasion.

Short, giving evidence to the Chilcot inquiry into the war, also said that the 2003 conflict had put the world in greater danger of international terrorism.

Declassified letters between Short and Blair released today show she believed that invading Iraq without a second UN resolution would be illegal and there was a significant risk of a humanitarian catastrophe.

She told the inquiry that she had a conversation with Blair in 2002. He told her that he was not planning for war against Iraq and that the evidence has since revealed that he was not telling the truth at that point, she said.

She also said she was "stunned" when she read the 337-word legal advice on the war written by the then-attorney general Lord Goldsmith during a cabinet meeting on 17 March 2003, three days before the war began. She was forbidden by Blair from discussing it during the meeting.

"I said, 'That is extraordinary.' I was jeered at to be quiet. If the prime minister says be quiet there is only so much you can do.

"I think for the attorney general to come and say there's unequivocal legal authority to go to war was misleading."

Short, who was applauded by some audience members in public seats at the end of her evidence, said the ministerial code was broken as cabinet colleagues were not aware of Goldsmith's modifications to his legal advice over the previous weeks. The inquiry has already heard how Goldsmith changed his mind over the need for a second resolution after visiting the US the month before the war.

Short said cabinet colleagues were unaware of the legal advice given by the most senior Foreign Office lawyers, Sir Michael Wood and Elizabeth Wilmshurst, which called for a second UN resolution.

"The ministerial code said legal advice should be circulated and it wasn't. We only had the answer to the parliamentary question [Goldsmith's short ruling]. There was a lot of misleading of parliament too by the prime minister of the day.

"The ministerial code is unsafe because it is enforced by the prime minister and if he's in on the tricks then that's it. When I found out what went into it I think we were misled."

She added that she had "various cups of coffee" with Gordon Brown, at that time the chancellor, who "was very unhappy and marginalised [in the run up to war]".

He was disillusioned about a number of issues, not specifically Iraq, and felt Blair was "obsessed with his legacy".

Later, Short added that after the war "Gordon was back in with Tony and not having cups of coffee with me any more".

Asked about the cabinet meetings in the run-up to the war, Short told the inquiry that the cabinet did not operate in the manner it was required to constitutionally.

"It was not a decision-making body. I don't think there was ever a substantive discussion about anything in cabinet. If you ever raised an issue with Tony Blair he would cut it off. He did that in July 2002 when I said I wanted to talk about Iraq. He said he did not want it leaking into the press."

Short described cabinet meetings as "little chats" rather than decision-making opportunities.

"There was never a meeting … that said: 'What is the problem? What are we trying to achieve? What are our options?'"

The declassified documents showed that Short believed the situation in Iraq to be "fragile" before hostilities began.

In one, written on 14 February 2003, she wrote: "Any disruption could lead to a humanitarian catastrophe. With some more time, sensible measures can be taken to reduce these risks and improve people's prospect of stability after the conflict."

Short told the panel that both the British and US armies failed to honour their Geneva convention responsibilities to keep order, describing the situation in the post-invasion aftermath as "mad", with food for refugees only being ordered at the last minute.

Short said Blair persuaded her against resigning on the same day as Cook by assuring her that the UN would have the lead role in reconstructing Iraq and that George Bush would support the creation of an independent Palestinian state.

Asked why she didn't resign earlier, she said: "If I knew then what I know now, I would have." As for the pronouncements that the French would not back a second resolution, it was one of the "big deceits" of the British, Short said.

The French president, Jacques Chirac, could have supported military action but not while UN weapons inspectors wanted more time and it should have been given.

"There was no emergency. No one had attacked anyone. There wasn't any new WMD. We could have taken the time and got it right. The forces weren't ready to go in. They have said that themselves."

Short ended her evidence by calling for a serious debate about the "special relationship" with the US, calling the current one "poodle-like".

Short stood down from the cabinet on 12 May 2003, nearly eight weeks after the invasion.

• Letter from Clare Short to Tony Blair on humanitarian planning and the role of the UN, 14 February 2003 (pdf).

• Letter from Short to Blair on the UN and US roles in post-conflict Iraq, 5 March 2003 (pdf).