David Cameron will fall 19 seats short of a Commons majority, according to a joint BBC/Sky/ITV exit poll.
The Conservatives would have 307 MPs, up 97 on 2005, Labour would have 255, down 94, and the Lib Dems 59, down 3. Nationalists and others would have 29.
That means Labour and the Lib Dems together could not have a majority.
There are reports of long queues of people still waiting to vote in some parts of the country after the most closely fought election in decades.
Polls closed across the country at 2200 BST but in Sutton Coldfield a BBC reporter says there are plans to lock voters inside the Mere Green Polling Station because the queues are currently so long. There are also reports of long queues in Sheffield, Leeds, Newcastle and other cities.
Lib Dem leader Nick Clegg has been to a polling station in his Sheffield Hallam constituency, where many are queuing outside, to offer his apologies to voters.
NOP and Mori surveyed 17,607 voters at 130 polling stations across the UK for the BBC/ ITV/Sky exit poll.
Prime minister
All exit polls have a small margin of error which could be significant in a tight election such as this one, in which the three main Westminster parties have been so close in the opinion polls.
There could also be different voting patterns around the country.
Nevertheless, Conservative leader David Cameron might find that winning one or two seats in Northern Ireland and forming a deal with the Democratic Unionist Party might be enough to give him a majority, making him Britain’s next prime minister.
The exit poll anticipates that the Labour Party has been more successful at holding on to its vote in seats in Scotland and Wales in seats where there is a large ethnic minority population and where there is an incumbent Labour MP standing again.
The poll also anticipates the Lib Dems will perform better in England than in either Scotland or Wales, but a fall in number of MPs would still come as a blow to Lib Dem leader Nick Clegg, who has been neck-and-neck in the polls with the other two main parties for much of the campaign.
It remains to be seen whether the poll proves to be accurate in its estimate of the Lib Dem performance – deputy leader Vince Cable told BBC News the outcome of the exit poll was "very strange" and insisted they had been "horribly wrong" in the past.
He said his party expected to have gained a lot from postal votes, as they were ahead in some polls when those votes were cast.
Hung Parliament
Labour’s election supremo Lord Mandelson described the election as a "cliff hanger" and refused to rule out trying to strike a deal with the Lib Dems, in the event of a hung Parliament, pointing that according to the rules Labour would have the first attempt at forming a new government.
"It is not the party which has the largest number of seats which has the first go, it is the sitting government," he told BBC News.
He also said that on the basis of the exit poll Britain’s first-past-the-post voting system was "on its last legs".
Electoral reform is one of the Liberal Democrats’s key demands.
Shadow schools secretary Michael Gove told BBC News "exit polls can show rogue results," adding: "We need to see the real results."
The vast majority of constituencies will conduct their counts overnight, with about 20 not due to begin the process until after 0900 GMT on Friday.
Most seats will declare in the early hours of Friday, but a handful of key marginals are expected to declare shortly after midnight, which could give an early indication of how the national result is going.
More than 44 million people are registered to vote in 649 Parliamentary constituencies. Elections are also under way for 164 English councils.
Polling in one constituency – Thirsk and Malton – has been delayed until 27 May because of the death of one of the candidates during the campaign. The exit poll assumes the Conservatives will win here.
Among the council elections taking place, voters will choose representatives in 32 London boroughs, 36 metropolitan authorities and 20 unitary authorities.
In these elections, a total of 15,785 candidates are contesting 4,222 seats.
Plane crash
Voting will also take place to choose mayors in Hackney, Newham, Lewisham and Watford.
In a separate development, Nigel Farage, a UK Independence Party Euro-MP, said he was "lucky to be alive", after being involved in a light plane crash near Brackley, Northamptonshire.
He is standing in the general election in Buckingham. He suffered non-life-threatening injuries when the aircraft came down just after 0800 BST.
He was initially taken to hospital in Banbury, Oxfordshire, before being moved to the John Radcliffe Hospital in Oxford. The pilot is currently being treated at University Hospital in Coventry.
Mr Farage’s agent, Chris Adams, told the BBC he had to be moved for more checks.
"I believe it’s just precautionary, obviously, for chest pains and we’ve just got to do all the tests… and the required X-rays, and hopefully he’ll be on the mend. The pilot, unfortunately, is in a more severe condition," he said.
A spokeswoman for the John Radcliffe Hospital said Mr Farage would be kept in overnight and "will be with us for a few days".
The crash is being investigated by the Air Accidents Investigation Branch and Northamptonshire Police. The aircraft was towing a banner when it crashed shortly after take-off.
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