The fallout: Heads will roll. But how many, and whose?

The Prime Minister is at Chequers today, putting the finishing touches to a reshuffle designed to restore his authority after what has been, by any reckoning, an appalling week.

But firing a raft of senior ministers to make way for the dwindling band of true believers is itself laden with risk. Asked what is the mechanism by which Blair could be driven from office, ministers point beyond the rubble of this Thursday's local elections to the Education Bill.

"People are already plotting," says another senior minister of Labour MPs, ahead of the final stage of Blair's schools reforms.

The Bill only passed its second reading with the help of Tory votes when 52 backbenchers voted against the Government. That figure could easily rise to 75, ministers fear, when the Bill returns to the Commons next month. And the ranks of Labour's disaffected MPs would be swelled still further by angry ex-ministers if Blair goes ahead with his reshuffle.

Should David Cameron find an excuse to vote with the rebels, Blair would be defeated and would have to leave No 10. With echoes of John Major sounding in Westminster after last week's "Black Wednesday", a minister said: "This Bill has every chance of turning into Blair's Maastricht."

But first Blair faces a tidal wave of recrimination over what all concede will be a dreadful set of local election results this week.

Ministers sent on rallying visits have found themselves in ever more "safe" Labour areas - a sure sign that the campaign managers have given up any hope of doing anything more than defend the party's town hall rump.

Indeed, in what could prove a highly significant coincidence, most postal ballots were received on Black Wednesday itself and on the following day. Canvass returns are almost universally described by senior Labour figures as "dreadful".

The blame game has already begun over who will be made to bear the greatest responsibility for the looming humiliation. The scapegoat list obviously includes Charles Clarke. A campaign based on Labour's crime-fighting prowess has had to be effectively junked a week before the poll.

But he shares the stocks. "When the dust settles, I wouldn't be surprised if the heat doesn't turn back up on Patricia," says one Labour aide. "NHS job losses have been terrible news."

Then there is Margaret Hodge, the Employment minister, blamed by many Labour MPs for "talking up" the BNP in an interview with The Sunday Telegraph earlier this month. Thanks in part to that interview, say MPs, the far-right party looks certain to add to its handful of council seats.

One Labour MP said John Prescott's affair was the least of the party's problems on the doorstep. "We have got a perfect storm. Three big negative news stories all collided at the same time."

But one figure alone bears the ultimate responsibility. Backbenchers are talking behind the scenes of gathering the 70 signatures that would enable a stalking horse candidate to stand against the Prime Minister, according to one Labour MP.

"You don't need a named stalking horse. All you need to do is to present the 70 names to show that you could trigger a ballot. I think the names are there," he said. "This has now reached a tipping point."

Blair has given an interview to today's News of the World. It was last night expected to conform to the PM's tried and tested strategy of emphasising his own resilience while laughing off the latest "worst week".

It is a formula even allies admit is wearing thin. A third-term government mired in personal sleaze, organisational corruption and, now, haemorrhaging its reputation for competence; suddenly comparisons with John Major's government seem oddly credible. Major lasted five years after his Black Wednesday; some Cabinet colleagues believe Blair might not last five months.

Charles Clarke, the Home Secretary

Crime: Foreign prisoners freed rather than deported

Alibi: Problem did not start on his watch

Survival rating: Low

Diagnosis: Fatally wounded

John Prescott, the Deputy Prime Minister

Crime: Had an affair with his secretary, Tracey Temple

Alibi: None

Survival rating: Fair, barring fresh revelations

Diagnosis: Permanent scars

Patricia Hewitt, the Health Secretary

Crime: Botching NHS reform

Alibi: Insists reforms are working

Survival rating: Good

Diagnosis: Badly bruised, no long-term injuries