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.