The SNP have accused Labour and the Lib Dems of colluding with Ukip to exclude them from the leaders TV debates, as David Cameron hinted he could agree to a showdown - but only with Ed Miliband.

The Prime Minister has threatened to boycott the debates unless the Greens are included.

But Downing Street sources have now suggested that the Tory leader still saw a "strong case for a two-party" head-to-head.

Senior Conservatives are nervous about the suggestion that Ukip leader Nigel Farage should participate in an TV debates.

They fear he will appeal more to Tory than Labour voters, harming Mr Cameron's chances of victory in May's general election.

The Prime Minister has in turn pressed for the inclusion of the Greens, who are more likely to take voters from Labour's left flank.

Labour, the Lib Dems and UKIP have jointly written to the broadcasters calling on them to move ahead with the debates even if David Cameron refuses to take part.

That suggestion, which could see the Prime Minister "empty chaired", would also exclude the SNP, Plaid Cymru and the Greens.

First Minister Nicola Sturgeon said that that would be a "democratic outrage".

"It's a simple democratic argument. The SNP is Scotland's biggest political party, we're also the UK's third biggest political party", she said.

"But I think probably the strongest argument is that we may hold a lot of influence in the UK Parliament after the General Election and if we do, then it's not just voters in Scotland who have got a right to hear what we have to say. Voters across the UK have a right to hear how we would use that influence if we had it."

SNP Deputy Leader Stewart Hosie said: "It's quite astounding that Labour are prepared to work with UKIP on a debate format which excludes the SNP, Plaid Cymru and Greens."

Earlier, the leaders of Labour, the Liberal Democrats and Ukip sent identical letters arguing it would be "unacceptable if the political self-interest of one party leader" prevented the debates from taking place

In the Commons Mr Miliband accused Mr Cameron of being "frit".

The Tory leader hit back calling Mr Miliband "chicken" for failing to back calls for the inclusion of the Greens.

Mr Cameron also set out what he said were the two different types of debate.

"There are two credible sets of debates," he said.

"You can either have a debate with all of the national parties who appear in this House. Or you can have a debate - or both - have a debate between the two people who'd become prime minister. Those are the credible debates."

Later the Green suggested that one broadcaster, ITV, had yet to decide who to invite to take part.

A senior No 10 source suggested discussions with the television companies would take place soon.

Asked if Mr Cameron had had a change of heart, he added: "The Prime Minister has repeatedly talked about the two scenarios; the multi-party debate - and he thinks the Greens should be involved - and he can see a strong case for a two-party one."

He also said that the SNP were "more than capable of making their case very robustly"

But he said that there was a "broader debate there about whether parties are represented nationally."

The suggested schedule is for debates on 2 April, 16 April and 30 April, ahead of the general election on 7 May.