Gordon Brown’s, leadership took a heavy blow yesterday as the once rock-solid seat of Glasgow East fell to the nationalists.

On Friday the PM will face a shell-shocked party high command to try to explain how labour came to lose its third safest seat in Scotland, and how he plans to reverse the party’s plummeting fortunes.

The loss of the seat was completely unexpected; the party was confident that it could mobilise enough of its predominantly working class voters in the city’s east end to hold the seat. But at 2.20am, after a recount, Mr Brown was informed that his party had lost out to the Scottish National Party who claimed a majority of 365, overturning a Labour majority of 13,500 at the last election. Turnout was 42 percent.

Fuelling unrest over Gordon Brown’s leadership, the result will ensure a hellish summer for the prime minister; virtually every other Labour MP in the country will be fearful that they will lose their seat at the next election.

For the SNP and John Mason, its candidate, the victory was spectacular. The nationalists are over-running Labour in Scotland. The by-election was prompted by the resignation on health grounds of Labour MP David Marshall.

On Friday at Warwick, cabinet ministers, union leaders and senior party figures will meet to discuss Labour’s next election manifesto – a meeting which the PM will be dreading.

The prime minister had told colleagues that “by-elections are by-elections”, dismissing suggestions that they are anything more than a chance for voters to register their unhappiness over a range of issues.

Mr Brown will come under pressure at Warwick from trade unions to take Labour to the left to reconnect with core voters. The defeat in inner-city Glasgow will intensify those calls.

Critics of the prime minister at Westminster have been desperately looking for some kind of “trigger” or “catalyst” to prompt a coup against Mr Brown, but it is far from clear whether Glasgow East is such a moment. “The cabinet should lead the coup but they haven’t got the bottle,” said one senior Labour MP. Alex Salmond, leader of the SNP, described the result as “an earthquake”.