Ed Miliband, Nick Clegg and Nigel Farage have joined forces to increase the pressure on David Cameron to sign up for televised general election debates.
The Labour, Liberal Democrat and Ukip leaders have sent letters to the Prime Minister warning that it would be "unacceptable" for him to block the clashes.
They are demanding Cameron - who last week made clear he would not participate unless the Green Party was included alongside Ukip - be "empty chaired" by broadcasters if he does not give ground.
The co-ordinated letters said the encounters during the 2010 election campaign "were watched by more than 20 million people and enthusiastically endorsed by all those who took part, including yourself".
"In recent days, you have announced that you are unwilling to take part in debates as proposed by the main broadcasters for the 2015 General Election," the missives added.
"I believe it would be a major setback to our democratic processes if these debates were not repeated in 2015 because of one politician's unwillingness to participate.
"I hope you will agree that the decision as to who should take part in the televised debates should not be in the hands of any party leader, each of whom inevitably has their own political interests to defend. It must be a decision independently and objectively arrived at."
The leaders insisted the broadcasters were under strict obligations of political impartiality and had made an "objective determination" about who should participate.
"While each of the other parties invited to take part in the debates has their own views on the proposal and the levels of participation offered and will continue to make their case in this regard, we all accept the independence and impartiality of the broadcasters and have committed to take part in the debates," the letters added.
"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.
"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.
"These debates are not the property of the politicians and I do not believe the public will accept lightly the prospect of any politician seeking to block them."
The BBC reported this morning that its impartiality guidelines would not prevent the debate going ahead without the prime minister.
A ComRes poll found yesterday that 55% of the British public thought Cameron was being "cowardly" for not participating.
Downing Street said its position on the debates "hasn't changed", insisting any multi party format must include the Green party.
Lib Dem Danny Alexander, Chief Secretary to the Treasury, accused the Conservatives of "wriggling and looking for any excuse it can" to get out of taking part.
He told BBC Radio 4's Today programme: "David Cameron, if he wants to put the kibosh on these debates, should pay a high political price for that."
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 to add 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".
"In 2010 David Cameron did very badly in these debates and was seen to be the loser. And I have seen this week that he has launched the six main priorities for the Conservatives' general election campaign and he has completely ignored the issues of Europe and immigration.
"I feel very strongly he would rather not debate these things with me on national television."