Cameron was accused of trying to "hold the debates to ransom" and "bully" broadcasters after he ruled out appearing in a televised one-on-one debate with the Labour leader.
Downing Street made a "final offer" to broadcasters on Wednesday night, saying the Prime Minister will sign up only to one 90-minute contest involving the Conservatives, Labour, Liberal Democrats, anti-European Union party UKIP, the Greens, the Welsh Plaid Cymru and the Scottish National Party.
Cameron's latest terms for taking part have been set out in a letter from his director of communications, Craig Oliver, to Sue Inglish, chairwoman of the broadcasters' leaders' debates committee.
It states: "In order to cut through this chaotic situation I am willing to make the following proposal: There should be one 90-minute debate between seven party leaders before the short campaign.
"In order for it to be organised in time, the debate should take place during the week beginning March 23. I will make myself available to negotiate the details. Having been the editor of numerous broadcast news and current affairs programmes, I know this is ample time to organise a programme."
"This is our final offer, and to be clear, given the fact this has been a deeply unsatisfactory process and we are within a month of the short campaign, the Prime Minister will not be participating in more than one debate."
Deadlock over the debates prompted Sky News and Channel 4 earlier to offer to host a head-to-head clash between the Prime Minister and the Labour leader on a date of the politicians' choosing, and Miliband said he was prepared to take part "any time, any place, anywhere" in an attempt to force the premier's hand.
Tories Scupper and Sink Debates
But No 10 sources said the "only real option" was one debate because time was running out ahead of the May 7 poll, and insisted the "fairest way forward" was with at least seven party leaders.
79% of you said in a poll that if a leader refused to take part in a TV debate you'd be less likely to vote for them https://t.co/GRZARML56Z
— Secular News UK & Ir (@SecularNews_UK) March 5, 2015
They branded the whole process a "shambles" and said broadcasters needed to "have a long hard look at themselves" over the way it had been handled.
Labour sources accused the Tories of doing everything they can behind the scenes to "scupper the negotiations and sink the debates".
.@David_Cameron why are you running scared of TV debates? The British people want a head-to-head TV debate. Let's give it to them.
— Ed Miliband (@Ed_Miliband) March 5, 2015
Douglas Alexander, who is leading Labour's general election strategy, said: "This is an outrageous attempt from the Prime Minister to bully the broadcasters into dropping their proposals for a head-to-head debate between David Cameron and Ed Miliband.
"That it comes only hours after Ed Miliband called David Cameron's bluff and said he would debate him any time, any place, shows the lengths David Cameron will go to run scared of a debate with Ed Miliband."
A Liberal Democrat spokesman said: "The public want these debates to happen and it is not right for one party to dictate their terms to try and gain an advantage.”
