If confirmed, the poll would be a stunning reversal for Mrs May, who had called the election on April 18 in a declared bid to strengthen her hand for the Brexit negotiations.
The country's leader was photobombed by a person dressed as the Sesame Street character as she cast her ballot with her husband Philip on the outskirts of Maidenhead, her constituency.
A hung parliament would have big effect for Brexit regardless of which party forms government with Labour pushing for a "softer" divorce from the EU.
The unexpected result would see Labour increase its number of seats by 34 to 266, while the Liberal Democrats would take 14.
"People have said they have had quite enough of austerity politics", he said, repeating his campaign promises to push for better funding for health and education.
Nicola Sturgeon's SNP appear to have suffered heavy losses.
The anti-Brexit Liberal Democrat Party did not make its hoped-for inroads.
So what is Elmo doing at Prime Minister Theresa May's polling station?
Labour deputy leader Tom Watson called the early results "very promising for Labour" and called Mrs May "a damaged prime minister whose reputation may never recover".More news: No-one has right to 'blockade' my country, says Qatar foreign minister
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While Jeremy Corbyn, the Labour leader, made a series of passionate and sure-footed performances in the final stages of the campaign, Mrs May was privately described by some Tory candidates as "Maybot" - repeating near-identical lines in her speeches.
The poll was produced in secrecy for the BBC, Sky and ITV by a team of eight leading political scientists led by John Curtice, a 63-year-old professor of politics from Strathclyde University in Glasgow. If she can't put together a government, other parties get the chance to have a go.
One of many key questions for the coming days will be what approach is taken to the Brexit negotiations, even if May stays as prime minister.
Early obstacles include the divorce bill, put by one estimate at up to 100 billion euros ($112 billion), and the status of European Union citizens living in Britain and Britons living elsewhere in the EU. She vowed to take the country out of the EU's single market and customs union, essentially a free-trade zone, radically changing Britain's relationship with one of its biggest trading partners.
But given the consensus expectation of a huge Tory victory in the popular vote, and of at least modest gains in the government's parliamentary majority, the exit poll numbers are a bit of a nasty shock (currency markets are already showing a drop in the value of the pound) to the conventional wisdom.
By contrast, Labour's Corbyn, a veteran socialist who had initially been written off as a no-hoper, was widely deemed to have run a strong, policy-rich campaign that enthused many followers.
May was criticized for a lacklustre campaign and for a plan to force elderly people to pay more for their care, a proposal her opponents dubbed the "dementia tax".
While the subject of Brexit was a main issue when the vote was scheduled, much of the campaign has focused not on Brexit, but on security, after high-profile attacks on public spaces in Manchester and London.
Corbyn said May's cut to police numbers had left the country vulnerable to attack, while May attacked the Labour leader over his record of voting against anti-terror legislation.