Ed Miliband, Nick Clegg and Nigel Farage have sent letters to David Cameron in a co-ordinated campaign to pressure the prime minister into joining televised general election debates.

The Labour, Liberal Democrat and Ukip leaders have warned Cameron it would be “unacceptable” for him to scupper the debates by refusing to take part.

They are demanding Cameron – who last week made clear he would not participate unless the Green party was included alongside Ukip – be represented on TV by an empty chair if he does not take part.

Ofcom, the commercial television regulator, has suggested in a consultation document that the Greens are unlikely to qualify as a major party as required for participation in the debate. The three main parties, plus Ukip, would qualify.

The broadcasters have drawn up plans for three debates: one between Cameron and Miliband, one between the leaders of the two main parties and Nick Clegg, and one debate between the three main leaders plus the Ukip leader, Nigel Farage.

Cameron has argued there is no logical reason for excluding the Greens from the debate. “You can’t have one minor party [Ukip] without having another minor party and I think that’s only fair,” he said.

The letters said in their co-ordinated letters that debates 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,” they wrote.

“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.”