In a setback to British Prime Minister Tony Blair, his International Development Secretary Clare Short on Monday resigned from the cabinet, accusing him of breaking promises over role of the United Nations in post-conflict Iraq.

"I am afraid that the assurances you gave me about the need for a UN mandate to establish a legitimate Iraqi government have been breached," Short said in her resignation letter.

She also accused the prime minister and Foreign Secretary Jack Straw of 'secretly' negotiating a UN Security Council resolution which contradicted promises she had given in parliament to MPs.

Short, who will be replaced in the cabinet by one of her deputies, Baroness Amos, telephoned Blair to tell him she could no longer stay in the government.

"She rang up the prime minister just after 10 am (1430 IST) this morning to resign," a Downing Street spokesman said.

Short's resignation comes just days after the controversial Labour MP missed a crucial House of Commons vote on plans for foundation hospitals and a subsequent cabinet meeting.

She had threatened to quit before the start of the war with Iraq, describing Blair as 'reckless' during the buildup to military action. But she decided to stay, saying that to leave on the eve of war would be 'copping out' when Blair had no option but to go ahead without a fresh UN mandate.

Short has caused controversy since becoming an MP in 1983 and she twice resigned from Labour's frontbench in opposition - including over the 1991 Gulf War.