Andrew Grice: Brown will not go quietly, but the air is thick with plots

As the voters of Glasgow East delivered their damning verdict on Gordon Brown, the Prime Minister took his wife, Sarah, to watch the Royal Shakespeare Company perform Hamlet in Stratford-upon-Avon. The timing was unfortunate. Yesterday, the air was thick with rumours of backbench and cabinet-level plots of regicide, as what some have called the "Shakespearean tragedy" of his political career took another dramatic twist.

"Glasgow East is the final piece of the jigsaw," one senior Labour MP told me. It proved, he said, that Mr Brown could not win anywhere. He had lost London in the mayoral elections, lost Crewe and Nantwich, come fifth behind the BNP in true-blue Henley and now lost a traditional working-class Labour seat – in his own political backyard, too. A former cabinet minister put it succinctly: "Gordon has taken us from being the one nation party to the no nation party."

To make matters worse, Labour was cautiously optimistic it would hold the Glasgow seat until the jitters struck on polling day. The crushing defeat will give Labour MPs – and cabinet ministers – plenty to think about as many of them start their holidays this weekend, including the likelihood that they will lose their own seats at the election.

It is not all over for Mr Brown yet, and he shows no signs of going quietly. There is unlikely to be an immediate move against him, as exhausted ministers run for the cover of their holidays and some thinking time.

But they could well mount an attempt at regicide when they return, possibly even before the Labour conference in September.

Critics of the Prime Minister sketch out three scenarios: an "orderly transition" in which cabinet ministers persuade him to stand down; a more damaging "disorderly transition" in which backbenchers take matters into their own hands by calling for him to go; and a "disorderly status quo" in which Mr Brown limps on to a general election defeat.

The phones of Labour MPs rang hot yesterday as they grimly compared notes. The conversations inevitably turned to the one factor that has prevented a coup so far: the absence of an obvious king over the water to succeed Mr Brown.

Alliances are forming, even at cabinet level, in a sure sign that the sands are shifting. Possible candidates are preparing for a contest, while insisting that this does not amount to plotting. The mood has changed; the tipping point may finally have been reached. Although there is backbench talk of a pain-free transition to Jack Straw, the Justice Secretary, or Alan Johnson, the Health Secretary, there would be strong pressure from the party to have a proper leadership contest this time, however distracting that might be.

In any case, David Miliband, the Foreign Secretary, who hesitated last year when he was urged by fellow Blairites to run against Mr Brown, would not hold back this time if Mr Brown quits. Brownites would urge their man's preferred successor, the Schools Secretary, Ed Balls, to stand against him, and others such as Harriet Harman, the Leader of the Commons, and the backbencher Jon Cruddas, might enter the race too.

Aides of Mr Brown insist he will not be forced out, dismissing as "fantasy politics" some of the more feverish speculation, which includes Sarah Brown urging him to resign. But one ally said: "The only circumstances in which he would go would be if he thought Ed Balls would get it."

Mr Balls would be a strong candidate but his closeness to Mr Brown might be a disadvantage if the Prime Minister is forced out.

With senior cabinet ministers calling for a "pause for breath", September could be the danger point for Mr Brown. He will try to keep the plotters at bay so he has a chance to stake his claim to lead the party into the general election at the Labour conference in Manchester. By then, he might have carried out a cabinet reshuffle, one of the few shots left in his locker.

During his summer break in Suffolk starting this weekend, the Prime Minister will doubtless think about some short-term policy initiatives to help hard-pressed families survive the economic downturn.

Yesterday, he spoke about the rising price of bread, milk and eggs, as allies admitted he had focused too heavily on his much-vaunted "long-term decisions", which had little relevance for ordinary people.

His autumn fightback plan will leave the "future agenda" to another day, in an attempt to help people through the immediate problems in a "fair" way, with measures targeted at those who need it most.

But this weekend, it is not certain he will get the chance to implement it.