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".
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.
In a statement, the broadcasters said: "BBC, ITV, Sky and Channel 4 remain committed to holding election debates in the general election campaign. Following meetings with the Conservative, Labour, Liberal Democrat and Ukip representatives, it has not been possible to come to an agreement on the original proposal put forward by the broadcasters in October 2014.
"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."Suggest a correction