Gordon Brown was dealt a devastating blow early this morning when Labour tumbled to an extraordinary defeat in the Glasgow East by-election.

John Mason, the SNP candidate and Glasgow councillor, pulled off a stunning victory, overturning a 13,507 Labour majority at the general election in 2005, to win by 365 votes from Labour’s Margaret Curran. Turnout was just over 42 per cent.

The result will send shock waves through British politics, adding to the Prime Minister’s mountain of woes and raising more inevitable questions about his political future.

It is the third such by-election humiliation for Mr Brown in as many months, following on from the loss of Crewe and Nantwich in May and last month’s lost Labour deposit in Henley.

However, the loss of the seat in the East End of Glasgow, previously regarded as a Labour fortress and the party’s third safest seat in Scotland, dwarfs even those two previous shock results and will concentrate the minds of Labour backbenchers worried about holding on to their own constituencies.

For the SNP, it rates as a spectacular victory at the very least on a par with its breakthroughs in 1988 in Glasgow Govan and in 1967 in Hamilton.

The Nationalists won the seat with an astonishing swing of more than 22 per cent, a turnaround that means few Labour seats in Scotland for either Holyrood or Westminster now appear secure from Nationalist incursions.

Alex Salmond, the SNP leader and Scottish First Minister, forecast a political “earthquake” in Glasgow East at the start of the by-election campaign and the result has fulfilled that prediction. Mr Salmond will claim that the voters of Glasgow East have also delivered a massive vote of confidence in his administration at Holyrood.

The victory comes on the back of the SNP triumph in last year’s Scottish Parliament elections when it won power, albeit as a minority government, for the first time in Scotland.

For Labour, the result is traumatic since most observers agreed that the party had a formidable candidate in the shape of Margaret Curran, a local MSP at Holyrood who has lived for part of her life in the constituency.

It will lead to inevitable speculation that the unpopularity of the Brown Government presiding over rises in food and fuel prices and the long-term economic downturn were the decisive factors in the minds of Glasgow East voters.

The Liberal Democrats and the Conservatives fought their own private battle for third place with the Tories taking it with 1,639 votes.

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