Britain is heading for a hung Parliament with the Conservatives the largest party, according to a BBC/ITV News/Sky News exit poll.

The poll gives the Tories 307 seats, Labour 255 and the LibDems 59.

Millions of voters have headed to the polls in the closest-fought General Election in years.

The three party leaders all cast their votes, with Tory leader David Cameron's arrival at his local polling station delayed by more than two hours after pranksters scaled the roof.

More than 44 million people are registered to vote for the 650 MPs of the new Parliament, with polling stations open from 7am until 10pm on Thursday. Voters have also been electing councillors in 166 local authorities across England.

There has been more uncertainty about who will eventually emerge as prime minister than in any election since 1992 - and there is a real expectation of a hung Parliament for the first time since 1974.

Mr Cameron's Conservative party needs 326 MPs for an overall majority in the House of Commons, a result that would propel him straight into Downing Street.

But even though a string of opinion polls on Wednesday night gave the Tories a clear lead over Labour and the Liberal Democrats, none had them gaining the level of support required for a majority administration. The polls all put the Conservatives in the lead with support ranging between 35% and 37%, with Labour on 28%-29% and the Liberal Democrats on 26%-28%.

On an even swing, the figures would give the Tories between 268 and 294 seats in the Commons. Labour could emerge with around 248-274 MPs, with the Liberal Democrats holding the balance of power on 77-82 seats.

It is the 100 or so Labour/Conservative marginal seats that hold the key to the result. And strategists believe that an unusually large number of voters will only make their minds up when they get into the polling booths.

No overall majority will mean a period of intense wrangling between all parties.

If voters choose to gravitate towards the two larger parties - despite Lib Dem leader Nick Clegg's recent surge in popularity - the balance of power could lie in the hands of the Welsh and Scottish nationalists or the Northern Irish parties.

In the event of a hung parliament, Gordon Brown is entitled to remain Prime Minister to see if he can command the confidence of the House of Commons.

But if Mr Cameron falls marginally short of an overall majority and beats Labour soundly, he may declare himself the winner - and Mr Brown would face intense pressure to resign.