The prime minister’s move, reported by the BBC and the Web sites of several British newspapers, surprised nobody, given Mr. Brown’s trademark as a political pit bull. He has a reputation for fighting his corner with a tenacity born of the decade he spent playing a smoldering second fiddle to Prime Minister Tony Blair before usurping him in a party revolt two years ago.

Through his aides, Mr. Brown put the matter as one of national interest. After a dismal winter of multibillion-dollar bank bailouts overseen by Mr. Brown, the aides said, and with early signs that the British economy might be past the worst of the recession, now was not the time to be replacing a prime minister the aides described as indispensable.

Mr. Brown’s prospects took a turn for the worse on Thursday night with the resignation of another cabinet member, the third in 72 hours and possibly the most serious for the prime minister. James Purnell, the 39-year-old work and pensions secretary, who is considered to be a standard-bearer for the party’s “Blairite” moderate wing, called for Mr. Brown’s resignation while announcing his own.

In his resignation letter, Mr. Purnell said that the prime minister’s departure would give Labor “a fighting chance of winning” a general election that is required in the next 12 months while Mr. Brown’s remaining in office would make “a Conservative victory more, not less likely.”

With two junior ministers having also stepped down, and Labor lawmakers circulating a draft e-mail message demanding that he quit, Mr. Brown’s position was increasingly being compared to that of Prime Minister Margaret Thatcher in November 1990, when cabinet resignations and a Conservative backbench revolt combined to oust her.

Mr. Brown’s next move is expected to be a cabinet reshuffle on Friday. But reports on Thursday suggested that Alistair Darling, the chancellor of the Exchequer, was threatening to resign if moved and that David Miliband, the foreign minister, was also resistant, further weakening Mr. Brown’s position.

Still, Mr. Brown held some trump cards, and the betting among Britain’s political commentators was that he would survive, if only narrowly, for reasons that had much to do with the procedural minefield facing his Labor detractors. Perhaps his strongest card is that unseating a sitting Labor leader is far more difficult, under the party’s rules, than it was for the Conservatives who dumped Mrs. Thatcher.

Mr. Brown’s fate is likely to become clearer by the weekend. While the overall pattern of results from the voting for many local and county councils is expected to be known by Friday, the results from the simultaneous voting for Britain’s seats in the European Parliament are not expected until Sunday. Pre-election polls have suggested that Labor could get fewer than 20 percent of the local election votes, trailing badly behind the Conservatives and the Liberal Democrats.

It was largely because of the expectation of dismal election results that the stirrings against Mr. Brown among his own members of Parliament had reached the point that a group of Labor backbenchers drafted an e-mail message telling him to go.

As reported in many British newspapers, the unsigned message paid tribute to Mr. Brown’s “enormous contribution to this country and to the Labor Party” since it took power in 1997. But, it added, “We believe that in the current political circumstances you can best serve the interests of the Labor Party by stepping down as prime minister and allowing the party to choose a new leader to take us into the next general elections.”

According to the newspapers, the dissidents planned to deliver the e-mail message, and make it public, as soon as Friday, if they could gather at least 70 signatures. They were quoted as saying they already had 50 backers, but they need a minimum of 70, the number required under party rules to begin the formal process of unseating a leader.

Projecting from the anticipated returns from the local elections, many Labor members fear they would lose their seats in a general election. But Labor’s rules also require the petitioners, in the absence of a resignation by Mr. Brown, to get at least 70 members to pledge their support to an alternative candidate.

That seems improbable because the health minister, Alan Johnson, the only person thought to be broadly acceptable as a successor to Mr. Brown, has said that he favors Mr. Brown’s remaining in his post and leading the party into a general election.

Third Minister Resigns From Gordon Brown’s Cabinet

Sir Alan, who was in the running for the post in 2000, told BBC News he would stand if he had enough support.

He added that he had “started to get indications of that from people in at least three parties”.

Earlier, Mr Martin announced he would stand down amid pressure over his handling of MPs expenses.

He told the Commons he would relinquish his post on 21 June, allowing MPs to choose his successor the following day.

‘Whiter than white’

Sir Alan, MP for Berwick Upon Tweed since 1973, said he was “willing to take on the task of leading reform” as speaker.

He told the BBC: “Over a period of time, even before recent events, various people have encouraged me to stand when the opportunity arose and I listened to that advice.

“But I think it’s very important now that the House of Commons has a Speaker who actually leads on reform rather than simply feeling that the job of the Speaker is to defend the House of Commons as it is.”

Sir Alan added that he cared deeply about making the House of Commons acceptable to the British public as the “core of our democracy”.

Asked whether he was “whiter than white”, he joked: “Nobody is whiter than white.”

Chairman of the Commons constitutional affairs committee, Sir Alan was a candidate for the Lib Dem party leadership in 1987.

Although his interview made him the first MP to express an interest in the post, a number of candidates have already been suggested as front-runners.

They include Tory grandee Sir George Young, Deputy Speaker Sir Alan Haselhurst, independent-minded Labour MPs Frank Field and Chris Mullin and Sir Alan’s former party leader Sir Menzies Campbell.

First MP discusses run as Speaker