SCOTTISH Labour leader Jim Murphy and shadow foreign secretary Douglas Alexander are on track for heavy defeats to the SNP at the General Election, new constituency polling has indicated.

Mr Murphy had a one-point lead in February in the East Renfrewshire seat he has held since 1997 but the new research, carried out by Lord Ashcroft between last Saturday and Thursday, finds he is now trailing the nationalists by nine points.

Shadow foreign secretary Douglas Alexander also has a mountain to climb if he is to avoid losing to 20-year-old student, Mhairi Black. A poll puts Ms Black 11 points ahead of Mr Alexander in Paisley and Renfrewshire South, up three points from February.

The SNP also holds commanding leads in Glasgow South West, East Dunbartonshire, North East Fife and former Liberal Democrat leader Charles Kennedy's seat of Ross, Skye and Lochaber.

David Mundell, previously the only Conservative MP in Scotland, is trailing the SNP by two points in Dumfriesshire, Clydesdale and Tweeddale. However, Lord Ashcroft found the Tories one point ahead of the SNP in Berwickshire, Roxburgh and Selkirk, with 30 per cent support. The incumbent, Liberal Democrat former Secretary of State for Scotland Michael Moore, is on 28 per cent.

The polls were released shortly after Scottish Labour launched their manifesto in Glasgow's east end, which came the day after Nicola Sturgeon was widely seen to have performed strongly in a televised BBC debate on with four other party leaders. Ms Sturgeon will launch the SNP manifesto on Monday.

Lord Ashcroft said that tactical voting may yet have a role to play in allowing pro-union candidates to hang on. With regard to Mr Murphy's constituency, he said: "This seat has a bigger Conservative share than many other Labour strongholds (25 per cent), and I found Conservative voters less likely to say they would rule out voting Labour (64 per cent) than would rule out the SNP (87 per cent). How many Tories will decide to lend their vote to Jim Murphy to stop the nationalists?"

Scottish Labour admitted that there was "no gloss" that could be put on the polls. A spokesman said: "If they are repeated on election day two things will happen - the SNP will have more MPs and David Cameron will walk back into Downing Street again."

However, SNP General Election Campaign Director Angus Robertson said that if there were more anti-Tory MPs than Tory MPs, Mr Cameron would be "locked out" of Downing Street. He added: "These constituency polls are very welcome, indicating SNP support continuing to grow across Scotland - encompassing areas which voted No as well as areas which voted Yes.

"The polls suggest that our policy to deliver jobs, growth and investment in services in place of Westminster cuts has huge appeal."