Tim Bale, professor of politics at Queen Mary University of London, said it's not even clear whether May will now lead those negotiations.
"She might start off doing that but the Conservatives might well replace her mid-stream," he said. "That's going to make it difficult for the EU 27 because they're going to want to know who they're talking to and what their policy is."
In the Conservative Party, recriminations were immediate and stinging. Many analysts said it was unlikely May could remain leader for long now that her authority has been eroded. Steven Fielding, a professor of politics at the University of Nottingham, called her "a zombie prime minister."
"Honestly, it feels almost like she is almost not aware of what has happened in the last 24 hours," Conservative lawmaker Heidi Allen told LBC radio. Allen said she couldn't see May hanging on for "more than six months."
The election's biggest winner was Corbyn, who confounded expectations that his left-wing views made him electorally toxic. A buoyant Corbyn piled on pressure for May to resign, saying people have had enough of austerity politics and cuts in public spending.
"The arguments the Conservative Party put forward in this election have lost, and we need to change," he said.
Initially blind-sided by May's snap election call, and written off by many pollsters, Labour surged in the final weeks of the campaign. It drew strong support from young people with the promise to abolish tuition fees, the hope of better jobs and a chance to own property.
"The young have a bad deal," said Ben Page, chief executive of pollster Ipsos MORI. "They didn't want to leave the EU. It appears clear they were determined this time to make a difference and vote."
Page said Corbyn, a lifelong left-wing activist who has spent decades speaking to crowds, was underestimated as a campaigner. While he was demonized by conservative newspapers, on Facebook Corbyn was trending.
Voter turnout in the election was up from 66 percent in 2015 to almost 69 percent, and half a million more young people registered to vote than before the last election.
"I felt passionate about voting to make sure Theresa May knew that young people like me would never support her or a Conservative government," said 23-year-old student Janet Walsh, who voted Labour. "I blame her party for destroying Britain by pushing for Brexit and austerity, two things that will ultimately be bad for my generation. This was the first time I voted."
From the start, an election called by May when polls gave her a commanding lead did not go to plan. She was criticized for a lackluster campaigning style and for a plan to force elderly people to pay more for their care, a proposal her opponents dubbed the "dementia tax."
Then, attacks in Manchester and London killed a total of 30 people and twice brought the campaign to a halt. They sent a wave of anxiety through Britain and forced May to defend the government's record on fighting terrorism.
It's unclear what role the attacks and their aftermath played in the election result. But the uncertain outcome is more evidence that after the populist surges that produced Brexit and President Donald Trump — and the centrist fightbacks led by Germany's Angela Merkel and France's Emmanuel Macron — politics remains volatile and unpredictable.
For many British voters, the feeling after the country's third major vote in as many years was weariness.
"We're in another mess again, and probably we're going to have to have another election, and it's all such a waste of time at the end of the day," said 85-year-old Londoner Patricia Nastri.