David Cameron has indicated that he will only agree to the televised leader debates if they take place earlier than suggested and also include parties from Northern Ireland.
This comes after the four major broadcasters proposed last week to hold three debates on April 2, 16 and 30, meaning they would take place during the "short" general election campaign, which begins on March 30 after parliament is dissolved.
In a joint statement, they heeded concerns expressed by the prime minister about the need for the Greens to be included if Ukip were featured in the TV debates. They set out plans for a 7-7-2 format, under which two debates hosted by BBC and ITV would feature the leaders of Tories, Labour, Liberal Democrats, Ukip, the Greens, Scottish National Party and Plaid Cymru, and a third on Channel 4 and Sky would pit Cameron against Ed Miliband in a head-to-head clash of the two leaders vying to be prime minister.
But now Cameron has told the media that he was concerned by the lack of Northern Irish parties. "As far as I am concerned that [Northern Ireland] is as important a part of our United Kingdom as Wales or Scotland," he said.
Asked on BBC Breakfast on Tuesday morning about whether their inclusion would allow the debates to go ahead, he said: "Yes I think the deal could be done, yes."
He later told Sky News that he also had concerns about the timing of the debates, lamenting that they "took the life" out of the 2010 general election campaign.
“I thought at the last election, they were excellent, the debates, but they took the life out of the election campaign. We know when the election is, in 100 days’ time, so let’s get on with the debates before the campaign begins," he said.
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.
Their last change appeared 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.
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."
They continued: "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."