The joint BBC/ITV/Sky survey released at 10pm after polling stations closed put the Conservatives ahead on 316 seats, with Labour on 239, the Liberal Democrats on 10, the SNP on 58 and Ukip on 2.
If the exit poll is correct, it looks likely Cameron will be able to remain as prime minister for second term. However all the other parties urged caution, with the Lib Dems and Ukip outright rejecting the forecast.
If the eventual results match the poll it would leave the Tories 10 seats short of an overall majority but ahead of its result in 2010. Labour would be down 19 seats on their result last time as the SNP were forecast to make sweeping gains of all but one Scottish seat.
Another poll conducted by YouGov for The Sun of 6,000 people who had voted painted a much brighter picture for Miliband, putting Labour and Tories tied on 34% each, Ukip on 12%, Lib Dems on 10%, the SNP and Plaid Cymru on 5% and Greens on 4%.
Election Results:Live Updates Throughout The Night
Conservative chief whip Michael Gove immediately moved to claim victory, should the exit poll prove correct. "If the exit poll is right, it means the Conservatives have clearly won this election and Labour have clearly lost it," he told the BBC.
Labour deputy leader Harriet Harman understandably urged caution over the poll. "I have been on television where I have been commenting on exit polls in the past where the exit polls were wrong, because it is difficult to read these situations from exit polls," she said.
Asked if Miliband could continue as leader if the poll proved correct, she said: "Well, at this stage we are just waiting for the counts and seeing whether or not David Cameron has got a House of Commons voted in which is prepared to give him a majority."
Former Lib Dem leader Paddy Ashdown said he thought exit poll, which suggests a disaster for his party, was wrong. He told BBC presenter Andrew Neil he would "eat his hat" live on air if the Lib Dems only got 10 seats.
And responding to the exit poll, a Lib Dem spokesman acknowledged the election was "unpredictable" but rejected the findings. A party spokesman said: "This exit poll does not reflect any of our intelligence from today or in the run-up to polling day. We will wait for the final results."
SNP leader Nicola Sturgeon tweeted: "I'd treat the exit poll with HUGE caution. I'm hoping for a good night but I think 58 seats is unlikely!"
Ukip deputy leader Paul Nuttall also rejected the exit poll. "We are going to get more than two seats tonight," he told the BBC. "We are going to do well in Kent we are going to do well in Essex. I do think your exit poll is wrong.
If Ukip has won just two seats, it suggests Nigel Farage has not won in South Thanet. But Nuttall said: "I think Nigel Farage has won and I think he has won by a bigger margin that what people think."
In 2010, the exit poll got the eventual result almost exactly correct. However the team behind the numbers have warned this year's poll may be less accurate. Professor John Curtice, who is analysing the results for the BBC, has cautioned that the break down in the two-party system has made the task more difficult.
“Once upon a time, you could concentrate on Conservative-Labour marginals, throw in a few Lib Dem seats and basically get it right,” he told Bloomberg. “Until now you could ignore Scotland. You can’t ignore it any more."
The exit poll is different than all the opinion polls conducted during the seemingly endless election campaign. While they asked people how they intended to vote, the exit poll asks people leaving polling stations how they have actually voted.
The first sign of how well the SNP is performing will be around 2am, when Gordon Brown's old seat of Kirkcaldy & Cowdenbeath is due. And at 3am we will find out whether Scottish Labour leader Jim Murphy has survived Nicola Sturgeon's nationalist surge.
From 3am we should also start to see how bad, or good, a night it is going to be for the Lib Dems. Several senior MPs including Ed Davey, Lynne Featherstone, Jo Swinson and Alistair Carmichael have tough fights. Nick Clegg, who faced a strong challenge from Labour, will find out if he has survived at 4.30am.
A key battle between Labour and the Conservatives is at 3.30am. Education secretary Nicky Morgan is hoping to cling on to Loughborough - a seat that traditionally swung the way of the eventual government.
Ukip's election night also gets underway at 3.30am, when the result is due in Great Grimsby. Nigel Farage's party has focused its efforts in Essex and Kent, however recent polls suggested the party had fallen back in many of its target seats.
General Election 2015
GENERAL ELECTION 2015