The former shadow Chancellor Ed Balls has said an astonishing exit poll would, if accurate, mean another general election in 2017.
The Prime Minister’s decision to call a snap election appears to have backfired spectacularly.
Mrs May went to the polls on the back of huge personal popularity and terrible ratings for Jeremy Corbyn.
The BBC/Sky/ITV poll suggested the UK was heading for a hung parliament, with Conservatives 12 seats short of the 326 they need for an absolute majority in the Commons.
The poll put Tories on 314 seats, with Labour on 266, the Scottish National Party on 34, Liberal Democrats on 14, Plaid Cymru on three and Greens on one.
And Mr Balls said the prediction suggested Britain could be heading back to the polls very soon.
He told ITV: “If this is correct we’ll have another general election soon.”
Other party figures have reacted cautiously.
Tory Defence Secretary Sir 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.”
And polling expert Professor John Curtice said the result indicated that Mrs May had failed in her bid to get a greater mandate for Brexit but he could not rule out the possibility that the Tories would still gain a smaller majority
He told BBC News: “It seems to me that unless the exit poll is incredibly wrong then the Prime Minister has failed to achieve her principal objective, which was that she was going to achieve a landslide, or at least a very big majority for her party in the next House of Commons, and thereby provide her with rather more wriggle room over Brexit.”
And the Liberal Democrats appear to have ruled out a formal, or informal, coalition with Labour.
Lib Dem president Baroness Brinton said her party could not work with either Labour or the Tories as both are pushing for a “hard Brexit”.
She told Sky News: “A coalition is not on the cards, not just because of the 2015 result but because of big policy differences.”