Comment: Even ministers whisper that the game is up

He will turn away empty handed any delegations to No 10 seeking his resignation in Labour's best interests.

The battle lines are thus drawn for a summer of discontent and an autumn of wretched uncertainty.

These are dreadful days for Labour. The seismic swing of 22 per cent recalls the by-elections that buffeted John Major's collapsing Tory administration into oblivion.

Ministers who grew up watching the destruction of the Major cabinet suddenly find that history has turned full circle and made them the hated monsters on the scaffold. In private, ministers and MPs, including Cabinet members, speak as though the game is up for the PM. But in public no heavyweight has yet broken ranks. This failure of nerve keeps Mr Brown safe for the summer. Blairite big guns like Alan Milburn, Charles Clarke and Stephen Byers may want a change but are desperate not to bloody their own hands, fearing the Brownites would rally the party against them to shore up their own positions.

There is sense in this: They treated Mr Brown as a pariah at the 2006 party conference after the coup against Tony Blair and do not want to give him a chance to do the same. Labour's rulebook does not favour a "stalking horse" type of challenge and the party has no tradition of regicide.

The only realistic way of forcing the PM out, in my view, is a delegation that would include at very least Justice Secretary Jack Straw, chief whip Geoff Hoon and Parliamentary Labour Party chairman Tony Lloyd. To be certain it would need a loyalist like Alistair Darling or Nick Brown too.

Some think it would work. One former minister says: "We can offer him a hero's farewell if he leaves quietly but if not, well, it will be very nasty."

However, there are rumours that crafty Mr Brown is thinking of making Mr Straw deputy prime minister in the autumn and there is talk of Mr Hoon being promised the European Commissioner job when Peter Mandelson stands down. Like a scarred prize-fighter waiting for the bell, Mr Brown can go to his seaside holiday retreat in the secure knowledge that most plotters will decamp to Tuscany and Turkey until conference.

Looking ahead, there are two periods of extreme vulnerability. One is the long gap between the House returning in October and the Queen's Speech in December if the conferences go badly. The other is the euro and local elections in June next year.

A disaster on the scale of the London elections would provoke panic. Some MPs talk about pre-empting that by installing David Miliband first and calling a snap election to coincide with the 9 June polls.

Behind all this speculation is an unbearable tension inside Labour. With every month that passes, David Cameron grows palpably stronger, amassing donations and media respect. Simultaneously, Labour grows weaker and the list of seats at risk gets longer. Mr Brown's reply is to wait two years for a general election in the hope it will turn around. After today's ghastly result, fewer will feel confident in his strategy.