The SNP won by 365 votes after a nail-biting recount, a swing of 23 per cent from Labour which previously had a massive 13,500 majority. It is now being predicted that if support drifted from Labour on this scale at a general election, Tory leader David Cameron would seize power with ease. The loss of Glasgow East will almost certainly spark a summer of Labour discontent over Brown's leadership, and may see him toppled by Christmas.
The SNP's John Mason won Glasgow East on a 42 per cent turnout.
Senior Labour figures and Cabinet Ministers are now planning a huge operation to shore up the PM - for the time being.
They will begin by going on TV and radio to defend Brown, explaining that he has been the victim of mid-term blues.
Party workers smelt defeat less than an hour after polls closed last night.
The first ballot boxes that arrived at the count showed the SNP beating Labour three to one.
By 11.30 p.m. Labour bosses were admitting they had been beaten.
The devastating result came after the PM escaped a leadership challenge by the skin of his teeth in May following a triple whammy of election defeats.
He is likely to avoid an immediate revolt as Labour MPs are on their summer holidays, meaning they cannot scheme in Westminster.
But it is almost certain he will face a crisis when Labour meets for its annual rally in Manchester in September.
The embarrassment for Brown is even greater as his own constituency, Kirkcaldy and Cowdenbeath, is just 50 miles from Glasgow East.
Meanwhile, Brown will meet US Presidential candidate Barack Obama tomorrow before heading for two weeks' holiday on the Suffolk coast and then going to the Olympic Games in Beijing.
He is already planning a mid-September Cabinet reshuffle.