Labour is to postpone the start of its Scottish leadership contest until after the forthcoming by-election in Glasgow East in a move that will be widely seen as revealing the depth of its anxiety about holding on to the seat.

Senior party sources confirmed yesterday that candidates to to succeed Wendy Alexander, who resigned as the Scots Labour leader on Saturday, will agree at a meeting of the Shadow Cabinet today to hold their fire until after July 24, the date pencilled in for the crucial by-election.

The procedures committe of the Scottish party's executive, which sets out the timetable for the leadership contest, will also back a postponement when it meets in the next 48 hours.

The main reason for the delay is Labour's need to draft as many activists, MPs and MSPs as they can into the constituency at a time when grassroots morale in the party is at a record low. The party also cannot risk a divisive Holyrood leadership campaign spilling over to the by-election.

Labour's fears were reinforced yesterday when Britain's most authoritative survey of political experts predicted that the party would lose the by-election to the SNP, which needs a 22 per cent swing to triumph.

The Phi100 panel's prediction will cause a shudder of fear in Labour from the Prime Minister downwards since it would be a devastating blow to the party and make Gordon Brown's position well-nigh untenable.

The politically balanced panel, which includes politicians and strategists from all the parties, has established a reputation as the most accurate predictor of election outcomes.

It was the first correctly to forecast that Boris Johnson would be elected Mayor of London and also accurately predicted the outcomes of the by-elections in Crewe & Nantwich and Henley. The majority of panel experts, 55 per cent, think that the result will be a SNP win with a narrow majority.

A further 10 per cent of the panel believe that the SNP will win comfortably. Labour in 2005 had a 13,507 majority in the seat.

Party sources said yesterday that the Scottish leadership candidates will be allowed to seek the necessary nominations while the by-election campaign is going on.

One well-placed source said: “We do not want to delay the process, but the campaign does not need to start now. Getting nominations does not distract from the by-election the way a full leadership campaign would.”

Another party source said: “We think we can get it done and a new leader in place around the time the Scottish Parliament returns after the summer recess in late August.”

The Scottish Conservatives were first out of the blocks yesterday when they named their candidate for the Westminster by-election as Davena Rankin, a researcher at Glasgow Caledonian University who stood for the party in Glasgow Cathcart at last year's Holyrood election.

The Tories, who came fourth at the 2005 general election, may struggle to hold on to their deposit. The writ for the by-election was formally moved at Westminster yesterday and the poll will take place on July 24. That date has sparked unease in the party since it falls in the Glasgow Fair trades holiday fortnight when many of Labour's core voters could be away from home. It will also be the first Scottish by-election held in July.

Alex Salmond, the SNP First Minister, used the issue to fire the first salvo of the campaign when he accused Labour of being in a “blind panic” over the Glasgow East poll.

“It is extraordinary and unprecedented for Labour to hold a Glasgow by-election during the Glasgow Fair - they are running scared of the electorate. Labour are clearly hoping to have a by-election with no voters.

“The deadline for getting a postal vote is July 9. People in Glasgow East must make sure that Labour does not succeed in disenfranchising them.”

Labour will select its candidate on Friday. It seems certain to be George Ryan, 45, the local councillor for the Shettleston ward, which forms part of the Glasgow East constituency.

The SNP, 24 hours earlier, is likely to select either Lachlan McNeil, an advocate, who fought the seat at the general election in 2005 or John Mason, who leads the SNP group on Glasgow City Council.

It was still not clear yesterday whether Andy Kerr, the former Health Minister at Holyrood, would stand for the leadership of Scots Labour. He was due to meet members of his constituency party last night.

If he does stand, he is likely to be joined in the race by Iain Gray, the former Enterprise Minister and Cathy Jamieson, the former Justice Minister.

Margaret Curran, the former Communities Minister, may stand for the leadership or deputy leadership if Ms Jamieson, the present deputy, decides to seek the top job.

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