Update 11.17pm: The DUP's Jeffrey Donaldson hailed the exit poll, saying his party would be "serious players" in a hung parliament, and vowed to lend its support to the Tories on issues such as Brexit and keeping the UK together.
He told the BBC: "This is perfect territory for the DUP because obviously if the Conservatives are just short of an overall majority it puts us in a very strong negotiating position and certainly that is one we would take up with relish."
Update 10.54pm: A Labour source said if the poll was correct it would represent the largest increase in a party's popularity during an election campaign "by miles".
But the source said it was "too early" to consider talking to other parties about the prospect of forming a government.
Former chancellor George Osborne said the exit poll indicated a "catastrophic" night for the Conservatives.
Mr Osborne, the former MP for Tatton and now Evening Standard editor, told ITV: "It is early days, it's a poll, if the poll is anything like accurate this is completely catastrophic for the Conservatives and for Theresa May.
"It's difficult to see, if these numbers are right, how they would put together the coalition to remain in office.
"But equally it's quite difficult to see how Labour could put together a coalition.
"It's on a real knife-edge."
Earlier: Conservative and Labour figures reacted cautiously to the exit poll predicting Theresa May could have lost her overall majority.
Tory Defence Secretary Michael Fallon told the BBC: "This is a projection, it's not a result.
"These exit polls have been wrong in the past."
Labour's shadow chancellor John McDonnell also warned against reading too much into the prediction, saying: "We have to have some scepticism about all polls at the moment."
Craig Oliver, former director of communications at Downing Street for David Cameron, told Sky News: "If this is true, if this is accurate in CCHQ there will be deep and lasting shock.
"It was the biggest gamble a politician has taken for a long time and if that exit poll is right, it's failed."
Award-winning filmmaker Ken Loach, a supporter of Jeremy Corbyn, added: "If this is true, it's an extraordinary result for Jeremy Corbyn's Labour Party, starting so far back and written off."
He said it would be recognition of Mr Corbyn as a "man of principle" rather than Mrs May's "robot" persona.
Northern Ireland Secretary James Brokenshire said it was too early in the night to be drawing conclusions but defended Mrs May's decision to call an election.
He told Sky News: "I think it was right because ultimately she was presented with a situation in the House of Commons, also the House of Lords of people wanting to frustrate the whole Brexit process."
Former Liberal Democrat leader Menzies Campbell said the poll results showed Mrs May's Brexit strategy had failed and insisted his party would be reluctant to make deals with other parties.
He told BBC News: "Tim Farron made it very clear, he said no pact, no deal, no coalition.
"We've had our fingers burned by coalition, I don't need to tell you that, so I find it very difficult to see how Tim Farron would go back on what he has already said and indeed to persuade the membership of the Lib Dems that a coalition was a good idea from our point of view."
Shadow foreign secretary Emily Thornberry said it was still early but told Sky News: "They have been right for the last 20 years or so, 30 years, so I think we're on the verge of a great result.
"Just think only seven weeks ago the hubris of the Prime Minister who was 20 points ahead, who wanted to have a blank cheque, she wanted to do whatever she wanted with the country with Brexit, with the economy, with our National Health Service and we said no and we meant it.
"And we put forward a popular manifesto with a leader of the party who has withstood the most extraordinary personal attacks, and has actually shown if anybody was strong and stable it was him.
"And this is a great result, if it's true."
Ms Thornberry said Mrs May should "consider her position" as she will have "manifestly failed" if the exit poll turns out to be correct.
On what Labour would do, she added: "We will see what happens next but if the Labour Party is called on to provide the next government, we will do so and do it in a unified way under a popular manifesto... with a leader who is strong."
Senior SNP MP Stewart Hosie was cautious about the result as he sought to play down the prospect of deals with other parties.
He told BBC News: "If this poll is correct it would still point to the SNP winning the election in Scotland, which is what we set out to achieve.
"I don't recall us ever voting for significant Tory policy in the past and it would be hard to see in the current climate with the austerity cuts, hard Brexit party, that we would want to support them in any way in this future parliament."