Such is the sea change that has gone on in Scottish politics in the past year, that the Scottish Nationalists will now be widely expected to take Glenrothes from Labour, even though they need a 14.25 per cent swing to do it.

That would have been an impossible dream for the SNP until very recently. The Fife area, which contains the Glenrothes seat, has been run by Labour for decades at Westminster, Holyrood and local government level.

But ever since last year’s Scottish Parliament elections, when they ousted Labour from power at Holyrood for the first time, the SNP machine has appeared unstoppable, putting the party well on the way to becoming the dominant force in Scottish politics.

Not only do the polls show that Alex Salmond’s Nationalist administration at Holoyrood remains widely popular in Scotland, despite being in power for over a year, but last month they achieved a significant victory when they won a by-election in Glasgow East, ousting Labour from the seat.

The swing the Nationalists required in Glasgow East was 22 per cent, five points higher that that required in the forthcoming by-election in Glenrothes. They already run the local council in Fife in coalition with the Liberal Democrats, and won the central Fife seat at last year’s Scottish Parliament elections.

To make the Glenrothes battle even more dangerous for Gordon Brown, the seat borders his own constituency of Kirkcaldy and Cowdenbeath, and is not far from where the Prime Minister has his constituency home.

If Labour and Mr Brown needed an even sharper reminder of how vulnerable the party now is in Scotland, a new poll yesterday showed that Labour would lose half if its 38 Scottish seats in the Commons at a general election to the SNP, as the backlash grows against Mr Brown’s government. Among the victims would be Des Browne, the Defence Secretary, and Alistair Darling, the Chancellor.

The view in Labour circles in Scotland last night was that the Glenrothes contest is not likely to be held until October or even November. That would give a chance to Mr Brown to launch his autumn fightback and, if he can, see off any internal Cabinet threat to his authority as Prime Minister. Labour is also looking for a new leader in Scotland, with the outcome of the election not known until September 13.

Labour will be praying that the SNP administration at Holyrood commits a major gaffe or runs into big political trouble before voters go to the polls in Glenrothes, in order to give them a fighting chance of holding the seat.

But amidst all the doom and gloom in Labour ranks about the party’s by-election prospects, there is also a major problem for the SNP.

Failure to take the seat from Labour would be viewed as a reverse and would allow Labour to claim that the Nationalist bubble in Scotland was on the way to bursting.

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