Robbie Dinwoodie and Torcuil Crichton

Robbie Dinwoodie and Torcuil Crichton The Glasgow East by-election, which will make or break the premiership of Gordon Brown, will be fought to the wire today with each of the main rivals for the seat admitting that the result is too close to call.

Labour, defending a majority of more than 13,500 from the last General Election, has been narrow favourite with the bookmakers to hold the seat made vacant by the sudden resignation of MP David Marshall on health grounds, but its SNP challenger has always claimed it as a close fight.

Frantic campaigning in the last day of the election was due to begin with 5am maildrops through letterboxes of identified supporters as both Labour and the SNP plan to flood the constituency with activists to get their vote out. Campaigning will then cease when polling stations open at 7am.

Last night, David Marshall, whose resignation on health grounds triggered the by-election, said he was "more than happy" to answer questions from parliamentary authorities following speculation about his controversial home-based office arrangements.

In an e-mail to BBC Newsnight, in response to queries about his parliamentary allowances, he said: "In 29 years as a member of parliament I have never claimed parliamentary allowances for rent, rates or utilities for my family home. Everything has been passed to the relevant parliamentary authorities."

Yesterday, senior Nationalist strategists said they believed they would take at least 42% of the vote, making the outcome impossible to call. "It will all come down to the differential turnout," said one senior SNP figure.

"Whose voters are most committed? Whose activists are most up for it? That will make the difference."

The Labour camp said despite the party being at rock bottom in the national polls that they are hopeful. "It is going to be tight, it is too close to call," said David Cairns, the Scotland Office Minister who has been in charge of the Labour campaign.

The General Election turnout in the seat was just 48%, but election officials took the high number of postal votes returned as a sign turn out may be higher than expected in a by-election.

Labour mounted a final push with a fighting speech from its candidate Margaret Curran, delivered on a street corner outside a bingo hall. Ms Curran told supporters: "A number of weeks ago I promised that the Labour fightback was about to start. The Labour fightback is under way and we are on our way."

John Mason, the SNP candidate, said he was calling time on the government in Glasgow East. "Labour have had their chance, they have had 50 years and they are out of touch. A vote for Labour will change nothing, it is only an SNP MP that can make a difference for the east end."

First Minister Alex Salmond added: "As the cost of living continues to rise, we have seen the impact SNP pressure can bring - we have seen that when the SNP wins, the people of Glasgow East win. Glasgow East can be the epicentre of a political earthquake."

LibDem candidate Ian Robertson spent the last day of the campaign focusing on a closure-threatened fire station and on a petition calling for it to be saved, while Tory candidate Davena Rankin insisted: "More and more people are turning to the Conservatives."