The major TV broadcasters faced a deluge of criticism and threats of legal action on Monday when they proposed to include Ukip, but not the Green party, in the planned TV leader debates in the general election next year.

The plans were rejected by David Cameron, the Liberal Democrats, George Galloway and the nationalist parties, suggesting the apparent fracturing of the three-party system is going to make it harder than in 2010 to secure a cross-party political consensus capable of seeing a repeat of the pivotal 2010 TV debates.

Sky, the BBC, ITV and Channel 4 set out a joint proposal that Nigel Farage should join Nick Clegg, Ed Miliband and David Cameron for one debate. In addition there would be one debate involving Clegg, Miliband and Cameron, as well as a single head-to-head between Miliband and Cameron as the two potential prime ministers.

There is already a separate joint proposal from the Guardian, the Telegraph and YouTube for the leaders to take part in an online debate to engage younger audiences.

The Green party says it is inexplicable for Ukip to be given a platform when it is the Greens who have had an MP for four years, and polled at a higher level for many years. The broadcasters point to the consistent high poll rating of Ukip, as well as its showing in the European elections.

Natalie Bennett, the Green party leader, told the Guardian: “We are deadly serious about taking legal action over this, and seeing how we can raise the necessary funds. The public want a serious debate in which they hear the full range of views, including a party that stands up against Ukip on immigration.” She said her party had proposed two debates involving the five parties and one between Miliband and Cameron.

The Greens won the rare support of Cameron, who is seen as the leader most wary about a repeat of the 2010 debates, but also the most concerned about the potential inclusion of Farage.

He said: “I am in favour of TV debates but you have got to make sure you can come up with a proposal everyone can agree to, and I can’t see how you can have a party in that has an MP in parliament, but not another party.”

Danny Alexander, the Liberal Democrat Treasury chief secretary, said: “I don’t accept that Liberal Democrats should be excluded from any of the debates. We’re a party of government, another coalition, a hung parliament is a potentially likely outcome of the next election, and so I think it is right that as last time, we should continue to have full Liberal Democrat participation in all the debates.”

Miliband, previously an advocate of three debates involving the three largest Westminster parties, said he regarded the proposals as a “positive step forward”.

He added: “I hope David Cameron is not going to put up false obstacles to these TV debates happening. Frankly he should be saying these TV debates will happen, they must happen during the campaign, for my part I’m going to make sure they do.”

Farage was the most enthusiastic. “The decision is better than it could have been,” he said. “It does at least recognise the increasing popularity of Ukip. However, if the political landscape continues to change we would expect and ask for inclusion in a second debate.”

The nationalist parties said the proposal reflected a media-driven obsession with Ukip.

The format for 2015 would see three different styles of debate. Sky News and Channel 4 would broadcast the head-to-head contest between Cameron and Miliband, which would be chaired by the former Newsnight presenter Jeremy Paxman, with Sky News’s Kay Burley introducing the programme and presenting post-debate analysis.

The BBC would broadcast a similar debate to the 2010 format where the Conservative, Labour and Liberal Democrat leaders would be pitted against each other. This would be presented by David Dimbleby. ITV would have a four-way contest between the Conservative, Labour, Liberal Democrat and Ukip leaders. This would be chaired by the ITV presenter Julie Etchingham. The inclusion of Dimbleby, soon to be 76, was criticised by Sky’s Adam Boulton who told MediaGuardian that he should have stepped down to allow a “fresh start”.

The three debates would be more spread out than they were in 2010, taking place on 2, 16 and 30 April.

As well as live studio audiences, comprising members of the public, questions would be provided to the leaders using social media to ensure the widest possible audience engagement. Each broadcaster would make their debate available live to all other outlets.

Privately, broadcasters involved in the proposals blamed the political parties and particularly the Conservatives for not sitting down to discuss proposals following the 2010 election. “The Tories just refused to engage,” said one, blaming the residual belief that the decision to take part alongside Nick Clegg last time could have cost the Conservatives a majority.

There is also the separate initiative, announced in May, for an online debate broadcast on Google-owned YouTube in conjunction with the Guardian and the Telegraph. The proposal focuses on engaging young people online and aims to break the TV monopoly on the pre-election debates in 2010. Chris Birkett, a spokesman for the Digital Debate Campaign, told Sky News the media industry was changing. “Just because broadcasters did it last time why should they do it this time?” he said. He stressed that with more than half of people under the age of 44 get their news from the web rather than television, using Youtube made sense. “It’s time politicians moved into the digital age.”

The Labour MP Tom Watson said an online debate would appeal to a wider audience: “There is a younger audience, a more networked audience, the sort of people who follow Question Time on Twitter but don’t bother turning on the BBC need a look-in this election as well.”

Watson said broadcasters should go ahead with the debates even if not all the party leaders agreed to take part.

“I don’t know whether the broadcasters would have the bottle to do that but they should,” he said. “You shouldn’t allow a prime minister of the UK to run away from holding himself to account.”

At the launch of the Guardian bid, Alan Rusbridger, its editor-in-chief, said: “The digital world has become an increasingly vital democratic tool and forum for debate, and it’s imperative that politicians understand and embrace the opportunities afforded to them by it.”