The prospect of a legal challenge is now looming, amid warnings that joint proposals from the BBC, ITV, Sky and Channel 4 fail to reflect the “truly unique situation” of post-referendum Scotland.
The SNP’s Westminster leader, Angus Robertson MP, said: “These proposals will be utterly unacceptable to any democrat.
“What the London-based broadcasters are proposing fails in their duty to their viewers in Scotland, and simply doesn’t reflect the reality of politics across the UK today.”
One debate would involve a head-to-head confrontation between Prime Minister David Cameron and Labour leader Ed Miliband, and another would also include Liberal Democrat leader Nick Clegg. The third would involve all three leaders, plus Mr Farage.
It means Nicola Sturgeon, who is expected to replace Alex Salmond as SNP leader and First Minister next month, would be left to participate in separate Scottish TV debates with only other Holyrood leaders.
The surge in the SNP’s membership to more than 70,000 since the independence referendum makes it the third biggest party in the UK, while recent polling suggests it will gain 40 per cent of the vote next May and become Scotland’s biggest party at Westminster.
It has emerged that even Mr Cameron has questioned why the Greens – who have had an MP since 2010, whereas Ukip secured its first elected MP only last week – are being excluded from the debates, if Mr Farage is involved.
Mr Robertson said: “The broadcasters have the cheek to say that their proposed format factors in ‘changes in the political landscape’ to justify including Ukip, entirely ignoring the fact that the SNP are now by far the third largest political party in the UK.”
The Green Party is considering taking legal action if it is excluded from the debates, which were introduced in 2010 and proved a hit with viewers, attracting an audience of more than 20 million.
The party’s UK leader, Natalie Bennett, said: “We’re certainly already looking at the option of legal action and I think there will be other parties who will be looking at the same option.”
Scottish Greens leader Patrick Harvie said: “These proposals are clearly inappropriate for Scotland, and cannot stand. These debates fail to reflect the truly unique situation Scotland now finds itself in.”
Asked about the prospect of a legal challenge, a spokesman for the SNP said: “We will consider our options a step at a time. At this stage we want to discuss these unacceptable proposals with the broadcasters.”
Mr Cameron said he favoured debates “in principle” but that there were questions over whether Mr Farage should take part if the Greens were excluded. The Prime Minister said yesterday there are “some questions” over the broadcasters’ plans.
He said: “Why have all the debates inside the election campaign, rather than spreading them out over a longer period? And, also, why include some parties and not other parties?”
Asked if he would be willing to debate with Mr Farage, Mr Cameron said: “I’m in favour of TV debates, but you’ve got to make sure you come up with a proposal that everyone can agree to.”
The blueprint from the broadcasters would see former Newsnight host Jeremy Paxman chair a one-on-one contest between Mr Cameron and Mr Miliband for Sky News and Channel 4. The Conservative, Labour and Liberal Democrat leaders would clash in a BBC debate presented by David Dimbleby. Finally, the trio would be joined by Mr Farage for an ITV programme chaired by Julie Etchingham.
Ukip’s Scottish chairman, Arthur Misty Thackeray, said: “We have had to fight tooth and nail to get ourselves accepted on to absolutely everything we have managed to get on, so I say to Angus [Robertson]: welcome to our world, mate.”
A spokesman for the Scottish Conservatives said the debates were for the general election, and would be broadcast “to all corners” of the UK.
He added: “The SNP is not fielding candidates in the vast majority of the country, and Nicola Sturgeon has no designs on being prime minister.”
BBC Scotland is writing to the leaders of the main political parties north of the Border inviting them to discuss setting up general election debates. The broadcaster will be proposing debates during the campaign which would be screened in each nation, and would also be available across the UK.
A BBC Scotland spokesman said: “In Scotland, the BBC is proposing a debate, in peak time on BBC1, involving the leaders of the SNP, Scottish Labour, the Scottish Conservative and Scottish Liberal Democrat parties.
“We have written to the parties to begin discussions about our proposals.”
Ukip came top in England in the European elections in May, taking 11 new seats for a total of 24, while the Lib Dems have been languishing in the polls.
Bookmaker Ladbrokes has made Mr Farage the 2-1 favourite to “win” the debates, ahead of Mr Cameron on 5-2 and Mr Miliband and Mr Clegg on 3-1.
The Ukip leader said: “The decision is better than it could have been.
“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.”
A Liberal Democrat spokesman said that the party would welcome debates on the same format – three debates between the three main leaders, over three weeks – as in 2010.
He added: “We do not accept the proposal that the Liberal Democrats, as a party of government, should be prevented from defending our record in one of the TV debates.”
Mr Miliband welcomed the proposals as a “positive step forward”. He said: “I hope David Cameron is not going to put up false obstacles to these TV debates happening.
“He should be saying they will happen, they must happen, during the campaign. For my part, I’m going to make sure they do.”
A joint statement from the broadcasters said the debates were “vital in engaging voters with the political process”.