By Nick Assinder
Political Correspondent, BBC News website
Tony Blair has faced plenty of demands for his resignation in the past and always seen them off.
Mr Blair's reshuffle may have made things worse for him
The latest are without doubt the most serious and may yet prove to be terminal.
And the statements from Downing Street and loyalist MPs like Stephen Byers and John Reid that people are out to "topple" the prime minister must rank as one of the greatest statements of the obvious ever.
It is now claimed 50 Labour MPs are willing to sign a letter, if one is eventually finalised, giving the prime minister just a few weeks to set out a clear timetable for his departure.
The underlying threat is that, if he does not, there will indeed be an attempt to topple him probably with a leadership election at this autumn's party conference - which many in the Labour party had already expected to be his last.
But it appears the main aim, excluding the "usual suspects" who do want to remove him as soon as possible, is to force the prime minister to spell out clear plans for that much-talked-of "stable and orderly transition" of power, presumably to Gordon Brown.
And the demands go well beyond that usual group of rebels who take every possible opportunity to demand his head.
Mr Brown may not have plunged in the knife during his weekend interviews - he knows what happens to "natural successors", like Conservative Michael Heseltine, when they become assassins.
But his words have still succeeded in intensifying the pressure on the prime minister to announce a detailed timetable for his resignation.
His repeated talk of the need for "renewal in government" and "a new generation of policies", his reference to "the next stage of my political career" and his statement that people were "looking to" the prime minister for the next move left little room for doubt about his ambitions.
But he went further, suggesting Mr Blair was already "talking to people" about the transition of power and that the Labour Party wanted that stable and orderly transition.
He even went beyond that, suggesting the party needed to listen to concerns coming from voters not only in last week's local polls but in last year's general election - that was immediately interpreted as a recognition that big issues such as the war in Iraq and some of the prime minister's public sector reforms were a factor in the party's performance and may need attention.
As for the prime minister, if he hoped his brutal cabinet reshuffle would succeed in overshadowing Labour's grim local election results and talk about his own future, he has been sorely disappointed.
In a number of respects, it has actually made things worse.
Stripping John Prescott of his departmental responsibilities but leaving him with his titles and sacking Foreign Secretary Jack Straw amid claims he had upset US president George Bush over Iran have angered many Labour backbenchers.
Similarly, promoting hardcore loyalists to the top cabinet positions and sending out messages that he has no intention of handing over power any time soon have also increased the frustration and anger from some backbenchers.
Much of it is expected to come to a head at a meeting of the parliamentary Labour party on Monday night, although history suggests discontent at those strictly-controlled meetings is often muted.
So the question now is how the prime minister reacts to all this dissatisfaction and - if it is another show of defiance as his aides are suggesting - whether that will again see off his critics or if, this time, it will be a different story.