A new poll on Wednesday- for the first time - predicted a hung parliament could emerge in the UK from the June 8 election, after a campaign that began with Mrs May's Tories looking at a landslide win.

Wednesday evening's debate was anticipated as a low-wattage affair, with both Mrs May and her Labour rival Jeremy Corbyn booking proxies to quarrel with the country's minor parties.

But Mr Corbyn decided at the last minute to capitalise on his surprising momentum in the campaign, and personally joined the debate.

May's boycott then became the political story of the day, despite her desire to focus on Brexit-related issues.

Liberal Democrat leader Tim Farron said Mrs May had made it clear to the electorate that "you are not worth (her) time" - and "if she can't be bothered, why should you?"

"How dare you call an election and then run away from the debate?" he said.

Greens co-leader Caroline Lucas said "the first rule of leadership is to show up - you don't say it's the most important election of our lifetime and not be bothered to show up".

May was also heckled online by an American TV show: House of Cards, whose Twitter account commented "@Theresa_May They respect you more when you show strength. Or show up".

Even The Sun, the Murdoch paper that has been fervently in Theresa May's corner, did her no favours by pointing out that May's proxy debater, home secretary Amber Rudd, had turned up despite the death of her father two days earlier.

Earlier in the day Mrs May defended her decision, saying she was taking questions from voters around the country instead of "squabbling" with rivals.

But the new distraction came on a day when a pollster, for the first time, predicted the Conservatives would do worse in this election than in 2015 - a result which would likely end May's prime ministership even if her party retained government.

YouGov's analysis of the latest polling data, applied to individual constituencies, came out with a prediction that Labour would gain almost 30 seats and the Conservatives lose 20.

This would result in a hung parliament.

The Conservatives would still be most likely to form a minority or coalition government that could gain the confidence of the Commons - but only just.

The prediction was hotly contested by other pollsters, most of whom are still predicting a dominant Tory majority of 100 or more seats, despite Labour's consistently improving poll numbers during the campaign.

And both parties reacted with skepticism, with one Labour insider telling The Times that Mr Corbyn's unpopularity would cause him to lose, not gain seats in the Midlands.

But YouGov stood by its modelling, which it said it had tested successfully in last year's Brexit referendum.

Wednesday night's TV debate had seven participants: Mr Corbyn and Ms Rudd plus the Liberal Democrat's Tim Farron, SNP MP Angus Robertson and the leaders of UKIP, Plaid Cymru and the Greens.

Ms Rudd summed up her opponents as the "coalition of chaos in action".

She scored a hit on Mr Corbyn when she pointed out that his parliamentary colleagues had passed a vote of no confidence in him, saying he had a weak team that could not be trusted to negotiate Brexit.

She also attacked Mr Corbyn's opposition to anti-terror legislation and his economic credibility, saying "there is no tax you don't want to rise??? we have to stop thinking, as you do, that there's a magic money tree".

But Mr Corbyn hit back with his strongest moment of the night, asking Mrs Rudd "have you been to a food bank? Have you seen people sleeping around our stations? Have you seen the levels of poverty that exist because of your government's conscious decision on benefits?"

His outburst won applause and cheers in the Cambridge venue.

The BBC was forced to defend its selection of a studio audience which appeared raucously pro-Corbyn, saying it had asked a polling company to choose a representative cross-section of the country's politics and demographics.