Polling stations across Britain closed at 10 p.m. (2100GMT) and ballots are being counted, with results due early Friday.
The Evening Standard editor, who was sacked from the Cabinet by Mrs May when she took office last July, told ITV: "Clearly if she's got a worse result than two years ago and is nearly unable to form a government then she, I doubt, will survive in the long term as Conservative party leader".
"What's happened is people have said they have had quite enough of austerity", he said, adding it was time for a government that is "truly representative of all the people of this country".
Theresa May has said Conservatives will act to ensure "stability" if the Tories are the largest party with the biggest number of votes, as expected.
Two of them - Hammond and Rudd - sided with David Cameron in opposing Britain's withdrawal from the European Union. Prime Minister Harold Wilson called another general election in October 1974, and Labour won a three-seat majority.
Rachel Sheard, who cast her vote near the site of the London Bridge attack, said the election certainly wasn't about Brexit.
The pound dropped about 1.6% to trade above $1.27 in the immediate aftermath of the projection, which was published as soon as voting ended at 5 pm ET.
Mohamed El-Erian, chief economic adviser at Allianz (Swiss: ALV-EUR.SW - news), said: "With initial exit polls pointing to the Tories (Conservatives) losing seats and that Prime Minister May's early election gamble is not paying off, markets are pricing in a more complex outlook for policy implementation, including Brexit".
If the poll is proved correct, a resurgent Labour will end up with 266 seats, the Liberal Democrats with 14 and the SNP on 34.
Heavy rains fall as ships, locals search Myanmar crash site
Myanmar's military says a transport plane with 120 people on board has disappeared while flying from southern Myanmar to Yangon . Local fishermen joined nine navy ships, five military aircraft and three helicopters in the search, the military spokesman said.
The conservatives took hit since announcing cuts for senior citizens May 18 in its election manifesto.
High-profile casualties of a night of shock defeats included Liberal Democrat former leader and ex-deputy prime minister Nick Clegg in Sheffield Hallam, Scotland's former first minister Alex Salmond in Banff & Buchan and the SNP's leader in Westminster Angus Robertson in Moray. Leaders in the Northern Ireland are expecting an alliance between the Democratic Unionist Party and Conservative Party to negotiate Brexit terms with European Parliament.
British Prime Minister Theresa May has failed to win a parliamentary majority in the United Kingdom election, leaving the country with what's known as a "hung parliament".
The result is another reminder of just how disillusioned voters are with the political establishment in Europe and the U.S. following Brexit, the election of Donald Trump and the implosion of France's main parties in the presidential election.
One of the campaign's revelations was the success of Mr Corbyn, who had been dismissed as unelectable by some members of his own party at the start of the campaign. It drew strong support from young people, who appeared to have turned out to vote in bigger-than-expected numbers.
The state opening of parliament by Queen Elizabeth II is set for June 19, when she will read out her new government's programme. Opponents denounced her as "weak and wobbly".
Two terror attacks later, and the political discourse in Britain has shifted to a debate centered around security, policing and how to put a stop to home-grown terror.
"She needs to consider her position", said Anna Soubry, an anti-Brexit Conservative lawmaker. "I've lost some remarkable friends".
The election throws up major questions about Brexit.
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