Nigel Morris: This could be the knock-out blow for a PM on the ropes

Crewe and Nantwich falls into Tory hands for the first time in half a century. Labour comes fifth in Henley. More than 300 Labour councillors are kicked out of office in town hall elections and Boris Johnson takes control of London. Labour's opinion poll support crashes to an historic low of less than 25 per cent. How could it get any worse for Gordon Brown?

But it did get worse in the small hours today when the SNP's stunning by-election victory in the former Labour fiefdom of Glasgow East was confirmed. And just as the Prime Minister should have been relaxing in his Suffolk holiday home, it could be about to get worse still.

He faces a torrid session at Labour's national policy forum today, where party activists and union leaders will be wondering what the point is of drawing up policies when they look doomed to election defeat. His meeting with Barack Obama tomorrow will also provide a cruel contrast between a president-in-waiting and a Prime Minister heading for the exit. Mr Brown will face a fresh round of questions over his chances of surviving until the end of the year.

Before the Glasgow East result was declared there were already subtle signs of Cabinet manoeuvrings. Labour backbenchers – many of whom had witnessed for themselves the mixture of hostility and indifference to the party on Glasgow's doorsteps – left for the parliamentary recess this week in disillusioned mood. The gloomy whisperings will intensify when they return from their Cornish beaches and Tuscan villas. Given the murderous mood of the electorate, even those Labour MPs with apparently comfortable majorities could be facing the dole in less than two years' time. They know there is little chance of an economic recovery boosting their electoral prospects as all the signs are that the slowdown will continue and perhaps worsen.

Many will wonder whether their desperate last throw of the dice could be to change leader. The mechanism for setting that process in train is unclear, requiring a succession of MPs to go public in their demands for Mr Brown to step down or for a delegation of party chiefs to tell the PM that his days are up.

Mr Brown is a famously resilient politician and his allies will close ranks around him. But it could be that the voters of Glasgow's East End have struck a fatal blow to his hopes of remaining much longer in No 10.