David Cameron has been accused of making "pathetic excuses" to dodge televised general election debates as opponents ramped up their attack on him over the issue.
The Prime Minister reiterated that the Greens must be involved in the sessions if Ukip are - and suggested the Liberal Democrats should have their role downgraded because they are only a "minor" party.
But in bruising clashes in the Commons, Ed Miliband deployed Margaret Thatcher's famous barb by branding Mr Cameron "frit".
The Labour leader pointed out that in 2010 Mr Cameron had condemned "feeble" reasons for backing out of debates, and said "no-one believed" his protests were genuine now.
"It is frankly a pathetic excuse. It is not for him, it is not for me, it is not for any party leader to decide who is in the debate. It is up to the broadcasters, that is the country we live in," Mr Miliband said.
"Is he really telling the people of Britain that he is going to seek to deny them the TV debate if he doesn't get to choose who is in them?"
Mr Cameron hit back that Mr Miliband was "chicken" for being unwilling to face off against Green Party leader Natalie Bennett as well as Lib Dem leader Nick Clegg and Ukip's Nigel Farage.
"I'm all for these debates but you cannot have two minor parties without the third minor party. Why is he frightened of debating the Green Party?"
The Tory leader indicated that he believed there should be two debates, rather than the three currently floated by the broadcasters.
"There are two credible sets of debates. You can either have a debate with all the national parties who appear in this House," he said.
"Or you can have a debate ... between the two people who would become Prime Minister. Those are the credible debates."
Pressed over whether the comments meant the Prime Minister was ruling out the mooted three-way debate with Mr Miliband and Mr Clegg, a No 10 aide said: "The PM has repeatedly talked about the two scenarios - if you are going to have a multi party debate he thinks the Greens should be involved and he can see a strong case for a two party."
Downing Street said discussions were "ongoing" and insisted the Prime Minister had repeatedly set out his views "very clearly".
But a senior Lib Dem source said dropping the three-way debate would be "unacceptable".
"It is just another excuse being used by David Cameron as he continues to run scared of the televised debates," the source said. "Next it will be an immovable hair appointment that he has got booked in ... Maybe he wants Grant Shapps asking the questions."
The PMQs exchanges came after Mr Miliband, Mr Clegg and Mr Farage sent identical letters to the premier insisting it would be "unacceptable" for him to block debates.
They demanded that Mr Cameron be "empty chaired" by broadcasters if he does not give ground.
"It would be unacceptable if the political self-interest of one party leader were to deny the public the opportunity to see their leaders debate in public," the letters stated.
"Therefore, if you are unwilling to reconsider, the three party leaders who have committed to participate will ask the broadcasters to press ahead with the debates and provide an empty podium should you have a last minute change of heart."
Mr Farage insisted there was no regulatory barrier to broadcasting without Mr Cameron.
"It is pretty clear that from the BBC's own editorial guidelines, you would be within your rights to hold these debates whether Mr Cameron turned up or not," he told Radio 4's Today.
He accepted that it would be "very difficult indeed" to have a session in which Mr Miliband was the only participant.
Citing regulator Ofcom's proposal for adding Ukip but not the Green Party to its list of "major parties" in support, he said it was wrong for one politician to seek to "deny the British people their right to see the issues discussed".
Meanwhile, Ms Bennett urged Labour, the Lib Dems and Ukip to call Mr Cameron's bluff by accepting that she should participate in a debate.
In a letter to the leaders, she wrote: "Staging the debates without the Prime Minister might score a point but would not serve the public, who rightly expect the political parties and the broadcasters to find a format that is acceptable to all concerned.
"In our discussion with ITV, they made it clear that they have not made a final decision on which parties to invite and would be prepared to change their current position in the light of fresh developments.
"If you indicated that you were open to the inclusion of the Greens, then I feel sure that ITV would respond.
"Having set out his objection to the current format, it would be hard for the Prime Minister to raise any new concerns, and this therefore gives the best chance of ensuring that the proposed Leaders' Debates can go ahead."
Shadow foreign secretary Douglas Alexander told BBC Radio 4's The World At One that Labour would not be urging broadcasters to involve the Green Party in the debates.
"I don't think we should get ourselves into the position, any of the parties, where we are either advocating or vetoing on behalf of our political opponents," he said.
Former Lib Dem leader Charles Kennedy suggested Mr Cameron's position could reinforce a feeling he was "out of touch" and a "toff".
"Actually, I think this has got a Bullingdon Club dimension to it," he said.
"I thought Labour were just silly and left-wing and student politics when they kept going on and on about that photograph of these posh toffs all in their hats and tails, behaving like idiots as undergraduates.
"And yet, you know, that contributed to this feeling which the pollsters have shown consistently ever since - this lot, Osborne, Cameron etc, 'out of touch', 'toffs' ...
"I just wonder if this will contribute to 'out of touch', and also 'running scared'."
Mr Clegg told Sky News: "My message to David Cameron is just 'Stop ducking and weaving, stop coming up with all these specious excuses'.
"The broadcasters have made a proposal, the rest of us are prepared to go along with those proposals.
"The British public enjoyed the leaders' debates last time, they want to see them happen again.
"I think this is a time just to get on with it."
A joint statement from the BBC, ITV, Sky News and Channel 4 said: "The broadcasters remain committed to providing election debates in the run up to the General Election.
"The debates played an important role in informing millions of our viewers in 2010 and we will continue to work with all the parties to ensure that they happen again in 2015."