By Nick Assinder
Political correspondent, BBC News website
Charles Clarke's criticisms of Tony Blair are, inevitably, being likened to the lethal verbal assault on then Tory prime minister Margaret Thatcher by her former deputy Sir Geoffrey Howe.
After being sacked from the cabinet in 1990, Sir Geoffrey delivered a devastating and calculated resignation speech in the Commons that no one doubts finished her off.
Clarke has questioned Blair's leadership
There are plenty of examples of former ministers plunging the knife into their bosses - enough to support the old phrase that a prime minister's opponents sit on the opposite benches in the Commons while his enemies are behind him.
So where do Mr Clarke's comments fit in this list and just how damaging are they to the prime minister?
The first such blow to Margaret Thatcher came when her Chancellor Nigel Lawson resigned after a row over her appointment of an adviser, Sir Alan Walters, whom he believed was undermining him.
When he finally snapped, in October 1989, he told MPs in his resignation speech: "The prime minister of the day must appoint ministers whom he or she trusts and then leave them to carry out the policy.
"When differences of view emerge, as they are bound to do from time to time, they should be resolved privately and, whenever appropriate, collectively."
Go for it
It was damaging but not fatal. The famous coup de grace came in November of the following year when the normally mild-mannered Sir Geoffrey Howe resigned in frustration at the prime minister's European policy and her dismissive treatment of him.
Sir Geoffrey finished off Margaret Thatcher
In one of the most electrifying performances of recent Commons history, he told a hushed House the prime minister's approach to policy was "rather like sending your opening batsmen to the crease, only for them to find, as the first balls are being bowled, that their bats have been broken before the game by the team captain".
He called on others to "consider their own response to the tragic conflict of loyalties with which I have myself wrestled for perhaps too long".
In other words, he told Michael Heseltine - long seen as Mrs Thatcher's most likely successor - to go for it. He did, and the rest is history.
The next Tory prime minister, John Major, had his own problems with his Chancellor, Norman Lamont, and the fallout from Black Wednesday, when Britain was forced out of the European exchange rate mechanism.
Famously, Mr Lamont used his resignation speech in May 1993 to declare the government gave the impression of being "in office but not in power".
This was not a fatal intervention, but it certainly set the tone for Mr Major's final troubled years.
Probably the most memorable resignation speech of the Blair era was that of the late Robin Cook after he quit the Cabinet over the Iraq war.
Again that was not a fatal blow, but it saw him putting a well argued case against what has continued to be a controversial decision to invade Iraq.
Rating Mr Clarke's comment that Tony Blair seems to have "lost his sense of purpose and direction" is not easy, but it seems closer to Norman Lamont's "in office but not in power" than Sir Geoffrey's call for a coup.
Lamont hit out at prime minister Major
What all these resignation speeches had in common was the fact they said out loud what many in the government and party were thinking at the time.
They managed to crystallise, in the words of a senior and respected figure, the wider criticisms of their respective prime ministers.
They offered some focus for the dissent and, in every case, they reduced the prime minister's standing and authority. Mr Clarke's statement almost certainly does the same thing.
Crucially, of course, it also comes as the prime minister's future is the biggest story in town, overshadowing just about every other development in politics.
And, while Mr Clarke insists he would be happy for Tony Blair to continue - if he can get back on track - his words will be seen as a further suggestion that it is actually time for a handover to Gordon Brown.
The prime minister dismisses all this as Westminster village chatter and shows his characteristic determination to ignore it all and plough on regardless, secure in the belief that his party is not prepared to bring him down.
That belief may or may not be well founded, but all eyes are now on this autumn's Labour conference as a pivotal event where the prime minister's future may well become clear.