Labour MPs from England are to be drafted in to the Glasgow East constituency as the party's by-election battle with the Scottish National Party reaches its decisive last three days.

About a dozen will be given the specific task of targeting disillusioned Labour voters who say that they are undecided on how they will vote on Thursday. Their involvement comes despite Labour strategists remaining optimistic and saying that the party still has its nose in front of the SNP.

But they are concerned about how “soft” the traditional Labour vote in this previously rock-solid seat has become and believe that the SNP could snatch victory if enough Labour waverers decide to throw in their lot with the Nationalists. “If too many of them switch, we could be in trouble,” a senior Labour source said.

Several junior government ministers will also be on the campaign trail this week but there will be no repeat of the tactic of sending Cabinet ministers to canvass but imposing a media blackout on their presence.

Labour believes that it holds a lead over the SNP of between 1,000 and 1,500 votes, far less than the 13,507 majority that the party chalked up at the 2005 general election.

“If we get good weather on the day, encouraging our people to come out, that could go up to nearer 3,000,” said another senior Labour source.

Labour is worried that Tory and Liberal Democrat voters, knowing that the two parties have no chance of winning, might vote tactically and lend their vote to the SNP.

If the Labour vote also collapses, that could be fatal for Margaret Curran, the Labour candidate. To guard against that, Labour has, since the first day of the campaign, been working on putting plans in place to get committed Labour voters to polling stations on Thursday.

Labour campaigners were set the target yesterday of knocking on 20,000 doors in the constituency to gather support.

Private polling by the SNP also shows that the outcome will be close, with the Nationalists saying that they are only four percentage points behind Labour. The party sent hundreds of activists into the last weekend of campaigning in Glasgow East and believes that the massive effort could be crucial in convincing thousands of undecided voters.

SNP strategists said that the main difficulty for them is convincing previously loyal Labour voters to “cross the Rubicon”.

One said: “When your family has voted Labour for generations, it's a big thing to turn your back on that tradition and be the first person in your family who deserted Labour. We know that it is difficult for some people and our job is to convince them to do it.” SNP tacticians also said that unlike other by-election shock victories and near-misses, such as Glasgow Govan in 1988 (won by the SNP with a 33 per cent swing) and Hamilton South in 1999 (when Labour's 15,878 majority over the SNP in 1997 crumbled to 556), the party has no “footprint” in Glasgow East as it had in these two constituencies.

“We had won previously in Govan [1973] and Hamilton [1967], so there was a history of people coming over to us in those seats but there's no such history in Glasgow East. That makes it harder, but it's definitely do-able,” a senior SNP campaign official said.

Alex Salmond, the SNP leader and Scottish First Minister, repeated yesterday his claim that a political earthquake was under way in Glasgow East.

He seized on an inside account of Labour's campaign by a volunteer worker, Brendan Perring, in The Sunday Times. Mr Perring quoted one of the party's campaign organisers, Martin Rhodes, as describing Gordon Brown's leadership as disappointing and predicting that the result would be “scarily close”. Mr Perring also described the mood of canvassers as “defeatist and downbeat”.

Mr Salmond said: “The Labour campaign is in chaos. Our campaign is optimistic and upbeat. The ground in Glasgow East is shuddering.”

David Cairns, the Scotland Office Minister, said: “I believe he [Brendan Perring] says he knows more about Swaziland than he does about Scotland, and it shows. If people want to come in, mislead people about their intentions, be taken into people's confidence and then betray that confidence for money, that is their lookout but we have an election to run.”

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