The three main party leaders were this afternoon anxiously awaiting to learn their fate as voters went to the polls to decide whether Tony Blair wins a historic third term in No 10.

A final Mori poll for the Evening Standard in London put Labour on 38%, five points ahead of the Conservatives on 33 per cent, with the Liberal Democrats on 23 per cent.

The figures would suggest another three-figure majority for Labour but with more than one in four voters saying they were still undecided, all the parties believe the final result could be much tighter.

Despite the convention that the parties do not campaign on polling day, Labour's deputy leader John Prescott used email and Tory leader Michael Howard hit the telephone in a final bid to rally the troops. Mr Blair was the first of the leaders to cast his vote. He was joined by his wife Cherie and sons Euan, 21, and Nicky, 19, who were voting for the first time in a General Election.

The Blairs were among a steady trickle of voters going to the polls in the village of Trimdon Colliery, in the Prime Minister's constituency of Sedgefield, Co Durham.

Liberal Democrat leader Charles Kennedy, accompanied by his wife Sarah, cast his vote in Fort William in his constituency in the Scottish Highlands.

And Mr Howard was joined by his wife Sandra and daughter Larissa as he voted at the Lympne Village Hall in his Folkestone and Hythe constituency.

Mr Blair, whose 52nd birthday is tomorrow, said ''Good morning'' to waiting reporters but made no comment about today's ballot. But with Labour desperate to persuade their supporters to turn out and vote, particularly in the marginal constituencies which form the key battleground of the election, there was no silencing Mr Prescott.

Labour circulated a mass email, in the name of the Deputy Prime Minister, urging the party's supporters to get out and vote.

In the email, Mr Prescott again argued that Labour supporters who fail to cast their vote, or who register a protest vote with the Liberal Democrats, would effectively be voting for the Conservatives.

That is an argument that Mr Kennedy has dismissed.

But in his email, Mr Prescott urged voters: ''Today you must choose between Labour, under Tony Blair, or the Tories under Michael Howard. A vote for the Liberal Democrats will effectively be a vote for the Tories.''

Mr Prescott cautioned: ''It takes time to design and build new schools, hospitals and town centres and it takes eight years to fully train a doctor. Similarly, it takes time to embed a strong economy and principles of social justice.

''But it doesn't take long to scrap, cut and destroy! The Tories could wreck many of Labour's achievements in a few short months.''

He went on: ''Even though the polls show Labour in front, it doesn't mean we'll win. When I was elected to Parliament in 1970, we were 16 points ahead and everyone assumed Labour had it in the bag. But we lost.

''Your vote is vital. Whatever you do today, vote. And if you want Labour, vote Labour.''

Mr Howard, meanwhile, hit the phones to rally would-be MPs. The Tory leader called candidates in target seats to offer last-minute encouragement from a Midlands phone centre.

Mr Howard said they all agreed the party was ''going great guns''.

Polling stations opened at 7am, with the electorate having until 10pm to cast their votes - and the first results expected around 11.30pm. After a month trailing the length and breadth of the country in the quest for votes, the three main party leaders know their fate now lies in the hands of the electorate.

Labour has led the opinion polls throughout the campaign - the final batch of eve-of-polling-day surveys putting them between three and six points ahead of the Conservatives.

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

The Conservatives believe they can take many of the seats where Labour has only a small majority, while the Liberal Democrats are also predicting that their opposition to the Iraq war will see them make big gains.

In the end, turnout could be crucial. The last General Election in 2001 saw a turnout of 59 per cent - the lowest since the troops returned home in 1918 at the end of the First World War - and there are concerns that there could be a similarly depressed vote this time.

There are fears among Labour strategists that disaffected supporters, unhappy over the Iraq war, could simply stay at home or cast a protest vote for the Liberal Democrats.

The uncertainty over the result has been heightened by the large numbers of undecided voters, with the final opinion polls suggesting more than one in four may be leaving it until the last minute to make up their minds.

The unpredictability has meant that - despite his huge Commons majority and seemingly comfortable opinion poll lead - Mr Blair has been warning supporters they are in a ''tough and tight'' race.

Aides have pointed to the 1970 election when the Conservatives, led by Edward Heath, ousted Labour Prime Minister Harold Wilson, even though he was the clear favourite to win.

In contrast, Mr Howard has been confidently predicting he can win, saying he hoped to secure ''many more'' than 50 extra seats, while Mr Kennedy has also been forecasting significant gains for his party.