Blair tried to placate his party's restive lawmakers by promising Thursday to resign within a year, but the harsh comments by Charles Clarke, formerly Britain's top law and order official, demonstrated the fragility of the truce.
Blair and Treasury chief Gordon Brown, widely expected to be the next prime minister, appeared to have reached a private understanding on the handover of power. But a sharp attack on Brown by Clarke — a Blair ally until the prime minister fired him in May — made clear the peaceful interval that party bigwigs had hoped for was unlikely to materialize.
Clarke harshly chastised Brown for the way he handled this week's party turmoil and said his succession as prime minister was not guaranteed. He said the grin on Brown's face as he emerged from Blair's 10 Downing St. office after handover talks with the prime minister had been provocative and poorly judged.
"A lot of people are very upset and cross about that. It was absolutely stupid, a stupid, stupid thing to do," Clarke said.
Clarke said Brown also should have acted more decisively against eight junior government officials whose protest resignations on Wednesday forced Blair's announcement.
"What he should have done was come out strongly and distance himself from them," he said. "He could have done that with a click of his fingers."
Brown's allies have insisted he was not behind the anti-Blair plotting.
Senior party figures have publicly backed Brown as the next Labour leader — and therefore prime minister — to try to prevent the eventual handover of power from highlighting intraparty divisions and throwing national elections expected in 2009 to a resurgent Conservative Party.
It was not clear what motivated Clarke's attack. Although he was long seen as a Blair ally, that relationship has grown chilly since his firing so his comments may not have been sanctioned by the prime minister's camp.
Blair's spokesman declined to comment on Clarke's remarks.
Blair "thinks the country wants the government to get on with the business of governing and that's what he'll be doing," the spokesman said, speaking on condition of anonymity in keeping with government policy.
Whatever Clarke's intent, his remarks made clear the amount of bitterness that has built up in the party during months of argument over the length of Blair's tenure.
Ruth Kelly, the communities secretary and a close Blair ally, said Clarke's statement was wrong and warned that continued sniping would be deeply damaging.
"I am sure there are others who share his view but what I would say is that I don't think that represents the center of gravity in the Labour Party, nor does it represent the majority view in the Cabinet," she told British Broadcasting Corp. radio.
She said the party needed a period of calm. "We have a duty ... to really refrain now for a period from this sort of intense political infighting," she said.
Blair had long resisted setting out a timetable for his departure as prime minister, fearing it would instantly drain his authority.
But he reluctantly announced Thursday that he would be gone by this time next year, when it became clear that was the only chance of ending days of damaging public turmoil. He refused to set a precise date for his resignation.
"I would have preferred to do this in my own way," Blair said Thursday, conceding that the Labour Party's annual conference this month would be his last.
Labour heavyweights had hoped that would be enough to calm the party after eight lawmakers quit their junior government posts Wednesday rather than remove their names from a letter demanding Blair's departure.
Anger over Blair's handling of last month's Middle East fighting and anxiety over the party's slide in the polls had fueled impatience in the party for him to leave quickly, or at least to say when he planned to go.
The low-level revolt raised fears that Labour's eventual change of command would be rancorous and messy — reminiscent of Margaret Thatcher's abrupt, involuntary departure from office in 1990 at the hand of rebels in her Conservative Party — rather than the "stable, orderly transition" that Blair has long promised.
The key question is whether the prime minister's new exit strategy will satisfy the party.
Early signals were that it would buy Blair time — but not much. He is eager to reach the 10-year anniversary of his 1997 assumption of office, which would be in May.
Speculation focused on May as the moment for his departure but his spokesman has said widespread guesses about a specific date were "just plain wrong."