The Prime Minister wants to campaign in Glenrothes in an attempt to help Labour hold on to the seat which is being strongly targeted by the SNP.

Mr Brown's change of heart was revealed yesterday by Jim Murphy, the new Scottish Secretary, who confirmed that the poll would be held on November 6, as revealed in The Times last month.

Mr Murphy said that Mr Brown, the economic crisis permitting, was determined to play a part in Labour's campaign and that he had decided to take part because it was the neighbouring seat to his own Fife constituency of Kirkcaldy & Cowdenbeath.

“He's a phenomenal and formidable campaigner, and he's determined to try to come here,” Mr Murphy said.

“It's important that he tries to do that. He knows that. I would like him to be here, he would like to be here, but his first responsibility is to get a solution to the international economic crisis,” he added.

Until now, it appeared that Mr Brown was inclined to abide by convention that the Prime Minister does not get involved in by-elections.Margaret Thatcher and John Major observed the convention, although Tony Blair did pay brief visits to two by-election contests during the early years of his premiership.

Mr Brown's absence during the Glasgow East by-election campaign in July, which saw a sensational SNP victory, provoked scathing private comments from senior Labour figures in Scotland who accused him of failing to lead from the front.

The Times revealed six weeks ago that Mr Brown was coming under increasing pressure from Cabinet colleagues to visit Glenrothes during the campaign. That pressure mounted when Lindsay Roy, the Labour candidate, said that he would be “surprised and disappointed” if the Prime Minister stayed away.

Mr Brown's new-found desire to campaign directly can be taken as a sign that Labour is becoming more optimistic about a positive result at the by-election.

He had a more successful party conference than expected and the chances of an imminent internal challenge to Mr Brown have receded in the face of the crisis caused by the global financial meltdown.

Labour is defending a 10,664 majority in Glenrothes and the Nationalists, who will field Peter Grant, the local council leader, require a 15 per cent swing to take the seat - less than the 22 per cent swing they achieved to win Glasgow East.

Labour strategists believe, however, that Mr Brown remains a popular local figure despite his recent political travails and that many voters in Glenrothes feel a personal and local loyalty to him.

The SNP has accused Labour of choosing November 6 in an attempt to bury a bad result in the media coverage that will flow from the result of the US Presidential election days earlier.

Mr Brown also faced accusations yesterdfay that he was attempting to turn the job of a government minister into a full-time campaigning role for Labour in the run-up to the by-election. The charge from the SNP centred on Mr Murphy's role in the campaign .

Labour said that a briefing given to Westminster journalists on Monday in which it was said that Mr Murphy would manage the campaign had been a “misunderstanding” and that the campaign would be led by Gordon Banks, the MP for Ochil, and John Park, a Fife regional MSP.

The odds on a Labour victory narrowed yesterday to 5-2. But bookmakers William Hill said that the SNP are still favourites to win at 2-7.

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