The Scottish National party is on course to inflict the final indignity on Labour by unseating Jim Murphy, its Scottish leader, according to a new opinion poll.
The survey, for the Tory peer Lord Ashcroft, adds further evidence of a looming SNP landslide north of the border in next month's general election.
Defeat for Mr Murphy in his seat of East Renfrewshire, which he held with a 20-point margin in 2010, would be a humiliation for the former UK minister who took over as Scottish Labour leader in December.
The latest batch of constituency polls for Lord Ashcroft found Mr Murphy trailing the SNP by nine points, a deterioration in his prospects since the same poll in February, which gave him a one-point lead.
The finding will reinforce claims by David Cameron that Labour cannot win a majority at Westminster on May 7. He says Ed Miliband would be forced to form "a coalition of chaos" with the SNP in order to enter Downing Street.
Nicola Sturgeon, the SNP leader, further raised the stakes in a television debate this week when she offered to work with Mr Miliband to "lock the Conservatives out of Downing Street".
Mr Miliband has rejected a coalition with the SNP, but has left open the prospect of a less formal deal. The Labour leader would be expected to turn first to the Liberal Democrats for support in the event of a hung parliament.
With Labour and Conservatives locked on about 34 per cent each in the polls, both big parties are privately being forced to contemplate attempting to form a minority government or working with smaller parties to try to assemble a coalition.
The release of Lord Ashcroft's poll on Friday cast a pall over Labour's release of its Scottish manifesto, which highlighted Mr Murphy's efforts to shore up support with pledges to address inequality, secure funding from the UK government and deliver greater devolution.
Lord Ashcroft also deepened the gloom surrounding the Lib Dems in Scotland, reporting that the SNP had widened its lead to 15 points in Ross, Skye and Lochaber, the constituency of Charles Kennedy, the popular former Lib Dem leader.
Danny Alexander, chief secretary to the Treasury and the Lib Dem candidate in Inverness, said he was "deeply sceptical" about the poll results.
"I think that in the end people will want to return hard-working Liberal Democrats rather than nationalists who want to go to Westminster to break up the United Kingdom," he said.
Lord Ashcroft also found the SNP ahead by two points against David Mundell, the only Conservative MP elected from Scotland in 2010.
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>However, he found some comfort for the Tories in the form of a one-point lead in the 2010 Lib Dem seat of Berwickshire, Roxburgh and Selkirk.In Berwickshire, the Tories were on 30 per cent, the SNP 29 per cent and the Lib Dems on 28 per cent, he said.
"Clearly, this seat could go any one of three ways, and all three parties will try to tell potential tactical voters that they are the only way to stop the other two," he said.
The findings will be scrutinised by tactical voting proponents, who say an SNP landslide would throw into doubt Scotland's long-term future in the UK, despite voters' rejection of independence in last year's referendum.
Mr Murphy's fate - and that of many of his Labour colleagues - could depend on whether Tory and Lib Dem supporters are willing to rally behind them to stop the SNP.
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