Broadcasters have published new plans for TV election debates including leaders of seven UK political parties.
The BBC and ITV plan to stage debates involving the Conservatives, Labour, the Lib Dems, Green Party, UKIP, the SNP and Plaid Cymru.
Sky and Channel 4's plan to host a head-to-head between Mr Cameron and Ed Miliband remains unchanged.
The broadcasters said the debates would go ahead regardless of whether any party leader refused to take part.
"The party leaders will be formally invited to take part in these debates. In the event that any of the invited party leaders decline to participate, debates will take place with the party leaders who accept the invitation," they said in a joint statement.
The BBC's chief political adviser Ric Bailey said the debates, set to last two hours each, would "go ahead" and the dates had been set.
Political correspondent Iain Watson
The broadcasters have gone out of their way - possibly at the expense of an engrossing spectacle for viewers - to accommodate Mr Cameron's stated desire to include the Greens in any line-up.
With the SNP and Plaid Cymru also invited to take part, it's just possible this would be the least damaging option for David Cameron.
The revised format comes after the broadcasters failed to reach agreement with the political parties over the original proposals, which only included the Conservatives, Labour, Lib Dems and UKIP.
Labour, UKIP, the Green Party, the SNP and Plaid Cymru all welcomed the initiative although the Lib Dems expressed misgivings and the political parties in Northern Ireland have objected to being excluded.
David Cameron had previously refused to take part unless the Green Party were included, saying "all national parties" should be represented - prompting a political row.
The Conservative Party said the new proposals "would be considered as part of the ongoing debates process".
The broadcasters - BBC, ITV, Channel 4 and Sky - said the proposed format takes into account representations from other political parties, polling and broad public opinion.
The suggested schedule is for debates to be held on 2 April, 16 April and 30 April, ahead of the UK-wide poll on 7 May.
The order of the debates is to be discussed with the political parties.
Broadcasters put forward initial proposals for live televised election debates in October 2014.
They proposed a head-to-head encounter between Mr Cameron and Labour leader Ed Miliband, another including Lib Dem leader Nick Clegg, and a third featuring the leaders of the Tories, Labour, Lib Dems and UKIP.
But in January, Mr Cameron objected to the Green Party not being included and said he would not take part - forcing a rethink from the broadcasters.
Under the new proposals, issued on Friday:
- there would be two seven-way debates including representatives of the Conservative Party, Labour, Liberal Democrats, UKIP, Greens, SNP and Plaid Cymru
- one of these would be hosted by BBC, and the other by ITV
- a third debate would include a head-to-head between David Cameron and Ed Miliband, jointly hosted by Channel 4 and Sky
- all debates would be held in April, during the general election campaign.
A Labour spokesman said the party would "debate anyone the broadcasters choose" and welcomed the proposal for the debates to be held during the official election campaign.
"The broadcasters have obviously made a very significant move to adopt wholesale the prime minister's proposals and it is surely now not possible for him to maintain his opposition to participating in these debates.
"We relish the opportunity for Ed Miliband to take on David Cameron directly in a head-to-head debate," they said.
But the Lib Dems said they would insist on being represented in all three debates.
"We have always been in favour of TV debates and are committed to making them happen but want to continue discussing the most recently proposed format," a spokesman said.
"We have always been clear that as a party of government, we must be able to defend our record in all the TV debates."
UKIP's Patrick O'Flynn said the broadcasters had "bent over backwards" to accommodate Mr Cameron's debate conditions but he had still not said he would take part.
The MEP told the BBC Mr Cameron was in "grave danger" of being seen as a "time waster" and someone who "has not got the courage of his convictions".
"It is not a good look for a prime minister to be seen running scared," he said.
Natalie Bennett, leader of the Green Party in England and Wales, said the broadcasters' revised proposals acknowledged that we now are "in an age of multi-party politics".
She told the BBC that the new proposals were "perfectly fair and reasonable" and those who disagreed should "simply get left out".
Scotland's First Minister Nicola Sturgeon also welcomed the news, saying the case for inviting the SNP was "unanswerable".
"The broadcasters have rightly continued to monitor the electoral landscape, taking into account the polling evidence, and the expressions of public support for the debates to include a wider range of parties - reflecting the reality of our democracy and electoral choices in Scotland and elsewhere."
But the proposals were met with dismay in Northern Ireland, where the DUP, Sinn Fein, the SDLP and the Alliance Party all said they should feature in some way.
"What is not acceptable is a situation where the largest party in Northern Ireland is to be excluded while regional parties from Scotland and Wales, with fewer seats in Parliament than the DUP, are to be included," DUP leader and First Minister Peter Robinson wrote in a letter to the broadcasters.
"The proposed decision is rightly seen as a further attempt by the broadcasters to marginalise Northern Ireland from the national debate."
Respect MP George Galloway is also pressing to be included in the debate.