The broadcasters have threatened to "empty chair" any political party leader who refuses to take part in live televised debates planned for the general election campaign.
In a joint statement, the four major broadcasters confirmed plans for a 7-7-2 format, under which two debates hosted by BBC and ITV would feature the leaders of Conservatives, Labour, Liberal Democrats, Ukip, the Greens, Scottish National Party and Plaid Cymru, and a third on Channel 4 and Sky would pit David Cameron against Ed Miliband in a head-to-head clash of the two men most likely to emerge as prime minister.
The broadcasters said that if any of the leaders decide not to participate, "the debates would take place with those who accepted the invitation".
That raises the prospect of a vacant podium - or "empty chair" - if any of the leaders refuse to participate in the programme.
BBC director general Lord Hall said: "There's nothing in any of our guidelines that says you can't empty chair anybody in any debate."
In an interview with the Radio Times he added: "You have always got to do what is right on behalf of the people who pay for you."
Proposed dates for the debates are April 2, 16 and 30 - with the final clash coming exactly a week ahead of the May 7 poll.
TV executives previously suggested three debates: one head-to-head between Mr Cameron and Mr Miliband, another also involving Lib Dem leader Nick Clegg, and a third with Ukip's Nigel Farage thrown into the mix.
The change appears designed to overcome David Cameron's refusal to take part in any debate that included Mr Farage but not Natalie Bennett of the Green Party - and to reflect a significant public campaign for the inclusion of the environmentalist party, as well as protests from the nationalist parties at their exclusion.
Following talks spanning three months with the main parties, the broadcasters have now issued formal invitations to the leaders to take part.
The empty-chair threat will increase pressure on leaders to participate in the televised showdowns, which were first staged in the 2010 general election campaign. It has traditionally been seen as strategically advisable for incumbent prime ministers to avoid TV debates with their rivals, which were considered to favour challengers.
But the move has been criticised by Northern Ireland's Democratic Unionist Party, which is the fourth largest group at Westminster, and Sinn Fein, which has five MPs, although they do not take their seats in the Commons.
The Lib Dems said they would also continue to push for Mr Clegg to be included in all three debates.
In a joint statement, the broadcasters said it has "not been possible to come to an agreement on the original proposal put forward" in October last year on the format of the debates.
The statement said: "Since October, the broadcasters have together and individually had a number of meetings and conversations with the parties invited to take part, the Conservatives, Labour, Liberal Democrats and Ukip, and also discussions and correspondence with the SNP, Plaid Cymru and the Green Party. All these discussions have been constructive and useful in informing our thinking about the debates.
"Over the three months since the original proposal was put forward, the broadcasters have also continued to monitor the electoral landscape, as we promised to do, taking into account the polling evidence, and the expressions of public support for the debates to go ahead and for a wider range of parties to be included in the debates.
"In view of these factors, the broadcasters are now inviting party leaders to take part in the following debates within the official election campaign and approximately two weeks apart.
"Two debates between the leaders of the following parties: Conservative, Labour, Liberal Democrat, Ukip, Green, SNP and Plaid Cymru. One of these debates to be produced by ITV, and one by the BBC.
"One debate between the leaders of the Conservative Party and the Labour Party produced by Sky and Channel 4.
"The proposed dates for the debates are April 2, 16 and 30. The order of the debates is to be discussed with the parties.
"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."
SNP leader Nicola Sturgeon said: "With a larger membership than the Lib Dems and Ukip combined, and more elected MPs than Ukip, the case for including the SNP in the televised debates was unanswerable."
Ms Bennett said: "The decision to include the Greens in two debates is an acceptance by the broadcasters that we now are in an age of multi-party politics."
She added: "The political landscape is fracturing and fewer and fewer people want the business-as-usual politics offered by the traditional Westminster parties. This is the Green Spring."
A Labour spokesman made clear that Mr Miliband is ready to take part in all three debates.
"We will debate anyone the broadcasters choose to invite and we are pleased to see the clear proposal to have three debates all in the general election campaign," said the spokesman.
"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."
The Tories would not be drawn on whether Mr Cameron would take part, with a spokesman saying: "These new proposals are being considered as part of the ongoing discussions about the debates".
A Lib Dem spokesman said: "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.
"We have always been clear that as a party of government, we must be able to defend our record in all the TV debates."
The DUP's leader, Northern Ireland First Minister Peter Robinson, has written to the broadcasters and Ofcom stating that the proposal is "unacceptable".
He said: "In light of the extension of the debates to smaller and regional parties, the failure of the broadcasters to invite the DUP to participate is inexplicable.
"The broadcasters' decision cannot logically or legally be defended.
Sinn Fein said it would challenge any attempt to discriminate against the party by excluding it.
A party spokesman said: "Sinn Fein will take every opportunity to present and promote its policies and positions."
