She will be replaced by Baroness Amos, a junior foreign office minister, who would be the first black woman to join the cabinet, according to a Downing Street source.

"She rang the prime minister just after 10:00 am (0900 GMT) this morning to resign," a spokesman for Blair said.

In her resignation letter, Short suggested Blair had broken assurances he had given her on the role of the United Nations in post-war Iraq.

Before the Iraq war, Short had threatened to quit if Blair went into the US-led war without UN backing.

"I think the whole atmosphere of the current situation is deeply reckless, reckless for the world, reckless for the undermining of the United Nations in this disorderly world ... reckless with our government, reckless with his own future, position and place in history," she said then.

"If there is not UN authority for military action or the reconstruction of the country (Iraq) ... I will resign from the government," she announced in a BBC interview on March 9.

But she broke her word and decided to stay afterwards when the war broke out on March 20, saying that to leave at that moment would be "copping out" when Blair had no option but to go ahead.

"I am very surprised and very sorry. She was excellent in her job," Labor lawmaker Glenda Jackson told the Sky News.

"On another level, I am not surprised. There was clear sniping because of what she said over the Iraq war," said Jackson.

Short has caused controversy since becoming a Labor lawmaker in1983. She twice resigned from Labor's frontbench in opposition, including one in protest against the party's backing for the Gulf War.

(Xinhua News Agency May 13, 2003)