LONDON — A humiliated Prime Minister Blair was forced to announce yesterday that he would stand down from his post within a year as he launched a desperate attempt to cling to power.

Only a week after promising that he would not give in to demands from Labour MPs to spell out his resignation plans, the Mr. Blair announced that he would be gone before next September’s Trades Union Congress.

But in a show of defiance to rebels who have demanded that he quit immediately, Mr. Blair refused to name the actual day on which he will leave Downing Street. After two days of feuding between Mr. Blair and Chancellor Brown, who is agitating to take over at no.10, the chancellor’s aides said he was broadly satisfied with Mr. Blair’s concession.

Rebel backbenchers suspended plans to send a delegation to no. 10 asking Mr. Blair to go. But last night, some doubted that the truce would hold, and many felt that Mr. Blair would be unable to hang on until May, his preferred time for standing down.

Brownite MPs and former supporters of the prime minister issued calls to Mr. Blair to clear up the uncertainty and leave before next May’s local elections in England and the Scottish Parliament and Welsh Assembly polls.

Union leaders preparing for their congress next week demanded that he go “as soon as possible.”

The general secretary of the private sector union Amicus, Derek Simpson, said Mr. Blair would receive a hostile reception when he addresses the congress in Brighton.

“Unless something changes, Labour will lose the next election,” Mr. Simpson said. The TUC general secretary, Brendan Barber, said: “It can only be a new leadership that can authoritatively articulate the new vision to inspire and re-energize the government’s supporters.”

Mr. Blair used a visit to a school in north London to make his announcement, accompanied by the education secretary, Alan Johnson. Joking about his unpopularity within Labour, he said: “I have brought Alan with me. You have got to have a friend. At least, I have got one.”

He added: “The first thing I would like to do is to apologize, actually, on behalf of the Labour Party for the last week, which with everything that is going on back here and in the world, has not been our finest hour, to be frank.”

Effectively saying that his power was draining away, he said he would have preferred not to be forced by his MPs and ministers to make the announcement. He said it was vital that the Labour leadership and MPs put the country above internal feuding. Making clear that the union congress and Labour Party conference this month would be his last, he said: “I am not going to set a precise date now. I don’t think that’s right. I will do that at a future date, and I’ll do it in the interests of the country and depending on the circumstances of the time.”

It is understood that a detailed timetable is being worked on in Downing Street. It would see Mr. Blair announce next February that he would step down as party leader on May 4 — three days after his 10th anniversary as prime minister and the day after the local elections.

A leadership contest would then allow his successor to be in place by mid-June.

President Bush’s spokesman, Tony Snow, emphasized Mr. Bush’s continued support for the prime minister. “Don’t count Tony Blair out,” Mr. Snow said. “Tony and the president have a lot of work to do. He’s still the prime minister of Great Britain. He’s a valuable ally.”

The Brown camp responded cautiously to Mr. Blair’s statement, saying it was “the first step on the road back to a stable and orderly transition.”

Supporters said they would not agitate for Mr. Blair to go on a specific date but would want more involvement for Mr. Brown on party issues and long-term policy. Mr. Brown broke his silence over the issue, making a statement on a tour of Glasgow.

He confessed to having differences with Mr. Blair over the succession but said it was time to “advance and get down to the business of the Labour government and doing our best by the people of this country.”

But opinion was divided on whether the conflict would die down.The Labour MP for Manchester Blackley, Graham Stringer, said: “Only the prime minister announcing that he is going very quickly and then commencing a leadership election is really going to take the pain out of this very difficult situation.”

The only declared candidate to replace Mr. Blair, John McDonnell, said the party would not accept a “backroom deal to install Gordon Brown as a proto-leader of the party in advance of them having any chance to cast their votes in a democratic election.”