The coalition of broadcasters bidding to stage the party leader debates is on the verge of unveiling a revised set of proposals in an effort to ensure that David Cameron will take part.

The prime minister objected to the original plan put forward by the BBC, ITV, Channel 4 and Sky on the grounds that the Greens were not invited to the debate that includes the Ukip leader, Nigel Farage.

Secret talks between the rival broadcasters have intensified this week as they sought to maintain a united front while looking to address Downing Street’s concerns.

TV news executives are said to be determined to go ahead with the debates but there is unease among some at the prospect of “empty chairing” the prime minister if he refuses to take part.

Cameron’s aides are said to be bent on ensuring he does not appear after the 2010 debates were regarded as a disaster for the Conservatives, fearing a repeat of the TV head-to-heads will boost the fortunes of Farage.

Negotiations between the broadcasters on Wednesday were focused on making a revised offer, likely to include opening up at least one of the debates to the Green party, leader Natalie Bennett, with a role also for Nicola Sturgeon, the leader of the Scottish National party.

But the exact form of the revised offer, which could be unveiled on Thursday or Friday this week, remains unclear and has not yet been put to the political parties.

The original plan, announced last October, consisted of three debates featuring Cameron, the Labour leader, Ed Miliband, and the Lib Dems’ Nick Clegg on the BBC, the same three leaders joined by Farage on ITV, and a head to head between Cameron and Miliband jointly hosted by Channel 4 and Sky News.

One news executive said “everything’s changed” as a result of Downing Street’s response, which prompted Miliband and the prime minister to accuse each other of “running scared” of the debates.

“What is being proposed has changed,” said the source. “All the broadcasters are now considering another offer.”

Negotiations took place between the broadcasters throughout Wednesday. The BBC’s politics chief adviser, Ric Bailey, was understood to be in meetings throughout the day trying to find a way forward acceptable to both broadcasters and political parties.

A senior news executive at the BBC said the corporation remained confident that Cameron would take part.

There was speculation that some politicians were unhappy with the choice of former Newsnight presenter Jeremy Paxman as the host of the Channel 4-Sky News debate, jointly fronted by Sky News anchor Kay Burley. But if the debates follow a similar format to 2010, the chair is not given the same leeway to pursue questions as Paxman was on the BBC2 show.

The attempt to come up with a revised offer follows Clegg call at the weekend for broadcasters to come up with other proposals after the prime minister refused to sign up.

Cameron said last week that the Greens should be included and suggested the three-way debate with Clegg and Miliband should not go ahead either. Another possible option would include the SNP in a separate debate broadcast only in Scotland.

A spokesperson for George Osborne said the chancellor was happy to take part in debates. They stressed that his position was the same as the prime minister’s, in that any debate should either include all minor national parties, including the Greens, or merely the two people likely to become chancellor, Ed Balls or Osborne.

“We have had no official proposal from any broadcaster so will take Ed Balls at his word,” they said.

“[Osborne] sees it as a good opportunity to remind people of the economic mess Ed Balls made last time he was in the Treasury.”

Separately, the Guardian, the Telegraph and YouTube have offered to host a digital debate with all five party leaders.

Broadcasters refused to comment on the discussions beyond a previously issued joint statement that said they remained “committed to providing election debates in the runup to the general election. The debates played an important role in informing millions of our viewers in 2010 and we will continue to work with all the parties to ensure that they happen again in 2015”.