The pound wobbled overnight after a shock projection forecast that May - who had been on course for a landslide just a few weeks ago - could actually lose her majority in the House of Commons after next week's vote.
May called the election three years early in a bid to strengthen her hand going into Brexit talks, which are due to begin on June 19.
Although the YouGov model has a wide margin of error, and was met with some scepticism in Westminster, it adds to polls showing the opposition Labour party gaining ground.
May refused from the start to take part in any head-to-head debate, and opposition Labour leader Jeremy Corbyn had said he would not take part without her.
But he changed his mind just hours before the live BBC hustings on Wednesday - and said May was treating voters with "contempt" by failing to join him.
Her decision to send senior minister Amber Rudd was "another sign of Theresa May's weakness, not strength", Corbyn said.
Tim Farron, leader of the smaller Liberal Democrats, told voters: "You're not worth Theresa May's time. Don't give her yours."
But May had earlier told a campaign rally that "debates where the politicians are squabbling among themselves doesn't do anything for the process of electioneering".
"It's actually about getting out and about, meeting voters and hearing directly from voters," the prime minister said.
The debate itself was a boisterous affair, covering Brexit, the economy, public services, climate change, immigration and security - a major issue after the Manchester attack last week.
Corbyn and Farron were joined by the leaders of the Green party, the UK Independence Party and Welsh nationalists Plaid Cymru and the deputy leader of the Scottish National Party.
Watch: We'll make sure there's a Brexit deal - Corbyn
Polls tightening
The Conservatives enjoyed a 20-point lead at the start of the campaign but this has now fallen into single figures.
"The movement in the polls over this campaign is bigger than in any election I've covered since 1945," veteran poll watcher David Butler said on Twitter.
The Daily Telegraph newspaper's rolling average of the last eight polls put the Conservatives on 44 percent, Labour on 36 percent, the centrist Liberal Democrats on eight percent and the anti-EU UK Independence Party (UKIP) on five percent.
But a shock projection in The Times newspaper forecast that the Conservatives could lose their majority in the 650-seat House of Commons.
Using new constituency-by-constituency modelling, the projection by pollsters YouGov said the party could lose 20 seats to end up with 310.
However, under the model - based on 50,000 interviews over one week - the Conservatives could get anywhere between 274 and 345 seats, meaning they could also increase their majority.
May said "the only poll that matters is the one that's going to take place on June 8", and insisted that only she had the plan to take Britain through Brexit.
'Liar, liar'
In the televised debate, Rudd accused Corbyn of having drawn up a "fantasy wishlist" manifesto paid for with a magic "money tree", and noting that most of his MPs backed a failed coup against him last year.
The Labour leader hit back by accusing the government of offering five more years of austerity "to fund tax handouts for the wealthy few".
SNP deputy leader Angus Robertson mocked May's claim to offer "strong and stable" leadership, saying: "Weak and wobbly is where we are."
A protest song branding May a liar was heading for the top of the charts on Wednesday - despite the BBC refusing to play it for fear of breaching electoral rules.
"Liar, Liar GE2017" by seven-piece band Captain Ska was released last Friday and currently stands at number three, according to the Official Charts company.
