Asked on Tuesday whether May would take part in televised debates, a Conservative spokesperson said: "our answer is no."

Sources near to May told the "Daily Telegraph" newspaper she had already undergone a face-to-face TV interview about her decision for an early election. The sources added said the choice at the upcoming election "is already clear." Downing Street issued similar comments to Channel 4 News, the Daily Mirror, and others.

Opposition leaders cry 'chicken'

"If this General Election is about leadership, as Theresa May said this morning, she should not be dodging head-to-head TV debates," Labour leader Jeremy Corbyn said on Twitter.

The Liberal Democrats called on broadcasters to go ahead with the debates, leaving an empty chair or podium where May would have stood. Nicola Sturgeon - Scotland's first minister and leader of the Scottish National Party - also called for a debate, with or without the prime minister. 

Liberal Democrat party leader Tim Farron addressed a public challenge directly at May.

"The Prime Minister's attempt to dodge scrutiny shows how she holds the public in contempt," Lib Dem leader Tim Farron said. "The British people deserve to see their potential leaders talking about the future of our country.

"I expect the broadcasters to do the right thing, don't let the Conservatives call the shots. If the Prime Minister won't attend - empty chair her - Corbyn can defend her position as they seem to vote the same on these matters. You have a moral duty to hold these debates."

Farron has sought to position his party as the clear opponents of leaving the EU - arguing that the Conservatives are dead set on a "hard Brexit" while Labour cannot make up its mind. 

"If you want to avoid a disastrous Hard Brexit. If you want Britain to have a decent opposition. If you want a Britain that is open, tolerant and united - this is your chance. You need to vote Liberal Democrat," Farron said.

Britain's late adoption of the TV debate

Should May duck a debate, this would mark a slight departure from her predecessor David Cameron.

Cameron took part in Britain's first ever televised leaders' debate in 2010 - then as leader of the opposition. Meanwhile in 2015, he ultimately agreed to a complex alternative format of debate, without a head-to-head element against his rivals, after first saying that he would not take part at all. This compromise was the upshot of severe criticism, not least from the Labour-supporting Daily Mirror tabloid, which sent a man in a chicken costume to shadow Cameron on the campaign trail in response to his reluctance to go head-to-head with then-Labour leader Ed Milliband. 

TV debates traditionally are seen to favor the underdog. The 2010 debate led to a large bump in the polls for then Liberal Democrat leader Nick Clegg, while both Nicola Sturgeon and UKIP's Nigel Farage were seen to have gained from a debate on ITV between all the major leaders except Cameron before the 2015 ballot.

May's stance is liable to come under continued pressure from her opponents - and from broadcasters hoping for some strong pre-election ratings - if she stands by it.

The Tories are up to 21 points ahead in some polls, so Downing Street might perceive a debate appearance as an unnecessary risk.