Leading article: Labour's Glasgow East defeat is a portent of worse to come

The catastrophe that was last night's by-election result for Labour had loomed ever since David Marshall MP announced his decision to step down. Glasgow East was the party's third-safest seat in Scotland and one of the safest in the country. Yet everything, from the political climate to the timing, conspired to make even a 13,000 majority seem marginal. In the end, Labour lost the seat to the Scottish National Party by 365 votes. They are votes that could decide the fate not only of a Prime Minister, but of his party.

That Gordon Brown is not minded to leave office quickly, if at all, was apparent from his speech to party activists in Warwick only hours after the result had been declared. The puritan in him seems almost to relish setbacks as a challenge to defy the odds. Saying that he did not want to wake up "24 months from now" to see tax cuts for the rich and health and education budgets cut was a pointed hint that he had no intention either of leaving office or calling an election before he has to.

Much can change, however, and change fast, even if the plotting that might be expected to follow such a loss is complicated by the parliamentary recess. How much pressure there will be on Mr Brown personally also depends on how far the quality of his leadership is held to account for Glasgow East, and how far he can pass the responsibility to outside – mostly economic – forces beyond national control.

All the signs are, though, that Labour lost Glasgow East largely on its record. And the catch here is that Labour, and Mr Brown personally, were once so keen to claim the credit for the good years that they will now find it hard to duck responsibility for the bad. Nor is it just the economy: a lengthening record of missed opportunities and mismanagement in areas as diverse as welfare, border protection and SATs has damaged Labour's boasts of competence.

The latest defeat also perpetuates the impression that Labour under Gordon Brown is somehow jinxed. It came in fifth at the Henley by-election, below the BNP. It lost Crewe and Nantwich to the Conservatives, and Boris Johnson was comfortably elected mayor of London, the most prominent of a slew of Labour losses in council elections across the country.

Perversely, this string of losses could even have the effect of prolonging Mr Brown's hold on office. Not only is there little appetite among MPs for another, potentially divisive, change of leader, but there is even less appetite for a quick election. Swings such as those registered in Crewe and Nantwich, and this week in Glasgow East, could see more than half the Cabinet evicted from their seats, including the Prime Minister.

Glasgow East, though, was not just about the Prime Minister and Labour. It was a drama in two parts, and the second part was the success of the Scottish Nationalists. To overturn a Labour majority of 13,000, even in such unpropitious economic circumstances, is a hugely impressive achievement, which testifies not only to the depth of disillusionment with Labour, but also to the appeal of the SNP under the adroit leadership of Alex Salmond. It also reflects the fact that, after decades of being taken for granted, Scotland's traditional Labour voters have a choice, and on Thursday more than 11,000 of them chose to exercise it.

There is thus a sense in which Labour's defeat in Glasgow East can be laid directly at the door of its devolution policy. Far from securing the future of the Union, this partial devolution has fostered the demand for even more autonomy. It has also, coincidentally, weakened the Labour Party north of the border to the point where its very survival as a credible political force is threatened. If the swing in Glasgow East were to be replicated across Scotland, Labour would be left with only one of the 38 seats it now holds.

The carnage in England would be less, because unhappy Labour voters, unlike their Scottish counterparts, would have no choice but to move to the right or stay at home. But this is scant consolation for the party, which would have to resign itself to long-term opposition without its Scottish MPs. England's Conservative majority would reassert itself.

Ten years after the establishment of the Scottish Parliament and the Welsh Assembly, it is not at all clear where devolution will lead; forecasts – wishful or otherwise – that it will spell the end of the Union may well be premature. The significance of Glasgow East is not that it brings closer the break-up of the United Kingdom, but it could presage the end of Labour as a party of British government. If it does, then the blame, for constitutional, as for electoral failure, will rest with the hapless Gordon Brown.