Voters were today going to polling stations around Britain as a Guardian/ICM poll predicted that Labour would win a historic third term, possibly with a majority of more than 100.

The poll, taken last night, found no evidence of a late swing that would deprive Tony Blair - who cast his vote this morning - of his third general election victory. It gave Labour 38%, the Conservatives 32%, and the Liberal Democrats 22%.

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In a uniform national swing, those figures would translate into a Labour majority of 130, although close contests in key marginals could mean that figure will be lower. Labour won the 2001 election with a majority of 165.

Polling stations opened at 7am today, with the electorate having until 10pm to cast their votes - and polls suggested many voters would need time to make a final decision.

On Monday, a Financial Times poll found 36% of voters had not yet made up their minds which box to cross. Those still undecided might find Guardian Unlimited's policy comparison guide a useful tool.

With a month of non-stop campaigning finally at an end today, the three party leaders returned to their constituencies to cast their votes.

Mr Blair was the first of the three main political leaders to cast his ballot.

Accompanied by his wife, Cherie, and his sons Nicky and Euan, who were voting in a general election for the first time, he walked to the polling station at 9.30am.

The Blairs posed for photographs outside the Trimdon Colliery community centre, in the prime minister's Sedgefield constituency, before entering the polling booth.

Like the Conservative leader, Michael Howard, and Charles Kennedy, the leader of the Liberal Democrats, Mr Blair, will spend an anxious day waiting for the first results, which are expected at around 11.30pm.

Meanwhile, the deputy prime minister, John Prescott, was still busy trying to get the Labour vote out, in breach of polling day convention.

Political parties do not campaign on polling day, but a mass-circulated email sent in Mr Prescott's name appealed to Labour supporters not to waste their vote.

Labour has led the opinion polls throughout the campaign, gradually widening its lead over the Conservatives over the course of the last month.

However, both the main parties have been saying that the outcome would be closer than the polls suggest, with Labour facing a tough fight to hold many of its key marginal seats.

The Conservatives believe they can take many of the seats in which Labour has only a small majority, despite the Guardian/ICM poll putting them one point down on their 33% result in 2001.

The Liberal Democrats are predicting that their opposition to the Iraq war will see them make big gains, and a result of 22% would mark the party's best performance since 1983.

Turnout could be crucial. The weather is expected to be fairly mild, if slightly damp, across much of the UK, handing reluctant voters a possible incentive to head to the polling stations.

The 2001 election saw a turnout of just 59%, the lowest since the end of the first world war in 1918. Labour strategists fear disaffected supporters, unhappy over the Iraq war and complacent about a Labour victory, could simply stay at home.

Mr Howard said he hoped to secure "many more" than 50 extra seats, while Mr Kennedy has also been forecasting significant gains for his party.