Gordon Brown is being lined up as a "secret weapon" in Labour's general election campaign in Scotland, in the hope he can replicate the dramatic impact he had on the independence referendum.

Party sources believe the former Prime Minister can help convince disillusioned voters to return to Labour.

Mr Brown has been credited by some as the man who "saved the union" following his intervention in the run up to September's vote.

But in the wake of the referendum his party has seen a dramatic slump in its fortunes, with opinion polls suggesting it is trailing the SNP by about 20 percentage points.

Some experts have predicted that that level of support could see Labour lose dozens of seats to the SNP in May, holding on to just four.

Polls on individual Scottish constituencies, due to be published next week, are also expected to make grim reading for Labour.

Tory peer Lord Ashcroft, who has carried out the surveys, recently described the SNP surge as "real".

Labour is hoping that Mr Brown can turn around its fortunes, winning over Labour voters who have failed to warm to Ed Miliband.

Sources said he was keen to see Mr Miliband, who worked for him for a decade, succeed, as well as new Scottish Labour leader jJm Murphy.

But it is thought that the ex-Labour leader, who is standing down as MP for Kirkcaldy & Cowdenbeath, will only be deployed in Scotland, and not south of the border, where he is less popular.

Sources said that the timing of any intervention was still being worked out.

There have been questions about whether Mr Brown should have been brought into both the independence referendum campaign, and the 2007 Holyrood elections, at an earlier stage.

There is also some concern that his high-profile position in the anti-independence campaign could turn off some Yes voters Labour is hoping to win back.

A Labour source described the former Labour leader as "our secret weapon".

He suggested that Mr Brown's interventions could involve "barnstorming" speeches of the kind Mr Brown gave the day before the independence vote, in which he called on those who opposed leaving the UK to "stand up and be counted", as well as appearances in television campaigns and other election material.

Another Scottish Labour source said: "During the election campaign he will be deployed in the hope that he can replicate the effect he had before the referendum."

He pointed to the impact that Mr Brown had in the 2010 general election campaign, when support for Labour increased in Scotland, despite dipping across the rest of the UK.

He admitted that that was a "high water mark" for Labour in Scotland, but added: "Gordon can get out there and help sell our message, just like he did in September".

Another Scottish Labour source said: "Gordon is a big beast, an ace card who will have a big impact."

"He'll be deployed very effectively to do the same kind of thing as he did in the referendum campaign," although he suggested that Mr Brown would be brought in at an earlier stage.

But he added that the details of Mr Brown's interventions were still "being worked out".

Earlier this week Mr Miliband told voters that backing SNP risked returning David Cameron to Downing Street, as he sought to frame the election as a straight battle between Labour and the Tories.

Mr Miliband is also under pressure from some within his own party to rule out a coalition with the SNP.

There have been predictions such a deal could effectively kill off support for Labour north of the border within a decade.

Angus Robertson, the SNP's Westminster leader, said: "This is a disastrous story for Jim Murphy - he was supposed to be the person who would turn Labour's fortunes around, but is obviously being discounted by his own colleagues after fewer than 50 days in the job.

"The SNP have just won a fantastic council by-election in Gordon Brown's constituency - Mr Brown of course isn't a candidate, and described himself as an ex-politician."