Gordon Brown faces a new by-election headache with the likely resignation of a long-serving MP.

David Marshall, MP for Glasgow East, is understood to be quitting for health reasons. In doing so he will leave the Prime Minister the challenge of shoring up Labour's vote in what should be one of its safest seats.

Any drop in the Government's majority would be seen as more evidence of a party in terminal decline. Last week Labour lost its deposit in the Henley by-election.

The party is planning an early poll, possibly as soon as July 24, in the belief that this offers Mr Brown the best chance of avoiding a poor performance, or a humiliating defeat.

The SNP promised that it would “throw the kitchen sink” at the seat and campaign on the high price of fuel, the state of the economy and Labour's leadership problems. Both parties are nervous at the prospect of Tommy Sheridan, the leader of the Solidarity Party, standing.

Speculation is mounting that Mr Sheridan, who lost his seat at the 2007 Holyrood election and who is facing a perjury charge over his court action against the News of the World, will be tempted back to politics.

Aemer Anwar, Mr Sheridan's close friend, said: “The matter is under consideration by Solidarity.”

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