DAVID Cameron is under mounting pressure to agree to a revised format to the TV election debates as the broadcasters warned that any leader who refused their proposals would be "empty-chaired".

Ed Miliband upped the pressure on the Prime Minister by quickly welcoming the "clear proposal" from the four main channels. The Labour leader insisted he would debate anyone the broadcasters invited.

"They have obviously made a very significant move to adopt wholesale the Prime Minister's proposals and it is surely now not possible for him to maintain his opposition to participating in these debates," insisted a Labour spokesman.

Tory HQ would only say the new proposals were being "considered".

The terse response came as the Conservatives published a new campaign poster showing Mr Miliband in front of Downing Street's famous black door, hugging the SNP's Alex Salmond with the words "your worst nightmare...just got worse". There are just over 100 campaigning days to go before polling day.

In a joint statement, the four major channels confirmed plans for a 7-7-2 format, under which two debates hosted by the BBC and ITV would feature the leaders of the Conservatives, Labour, the Liberal Democrats, the SNP, Ukip, the Greens, and Plaid Cymru, and a third on Channel 4 and Sky would pit Mr Cameron against Mr Miliband in a head-to-head clash of the two men most likely to emerge as PM.

The proposed dates are April 2, 16 and 30; the order is to be discussed with the political parties.

After three months of talks, the broadcasters made clear that if any of the leaders declined to participate, "the debates would take place with those who accepted the invitation". Lord Hall, the BBC's Director General, stressed: "There's nothing in any of our guidelines that says you can't empty chair anybody in any debate."

But the new format sparked fresh controversy with the Democratic Unionist Party, Sinn Fein and George Galloway's Respect Party complaining about their exclusion.

Mr Galloway threatened legal action. "My lawyers are on this now and we will be writing to the broadcasters, the watchdog Ofcom and the Prime Minister, if necessary, so these debates don't go ahead without me," said the Bradford West MP.

The initial proposal was for a 4-3-2 format. The first would have involved Messrs Cameron, Miliband, Clegg and Farage; the second the Tory, Labour and Lib Dem leaders and the third the Prime Minister and the Leader of the Opposition.

But Mr Cameron complained, saying if Ukip were invited to participate, then so too should the Greens.

Chancellor George Osborne then suggested if one so-called minor party was allowed into the leaders' debates, then all of them should be.

Labour and the Liberal Democrats accused the Tory leadership of "chickening out" and insisted it was not for a politician to say who should be in the debates; it was a matter for the broadcasters.

But the SNP cried foul, pointing out how it was now the third largest political party in the UK and had six times the number of MPs than the Greens. It made clear it was not ruling out a court challenge.

Following the broadcasters' announcement, Nicola Sturgeon, the SNP leader, welcomed the revised plans, saying: "With a larger membership than the Lib Dems and Ukip combined, and more elected MPs than Ukip, the case for including the SNP in the televised debates was unanswerable."

Natalie Bennett, leader of the Green Party in England and Wales, said: "The decision to include the Greens in two debates is an acceptance by the broadcasters that we now are in an age of multi-party politics."

However, the new format will not be to the liking of Nick Clegg as, unlike in 2010, the Lib Dem leader will not be part of a three-way debate with Mr Cameron and Mr Miliband.

A party spokesman made clear the Lib Dems were committed to making the debates happen but wanted to discuss the proposed format further, adding: "We have always been clear that as a party of government, we must be able to defend our record in all the TV debates."

BLOB

The SNP will launch its General Election campaign proper on Saturday February 7, party insiders have indicated.

The Nationalists are currently going through the selection of the party's General Election candidates, buoyed up by polls that suggest they could secure the majority of Scotland's 59 seats. But it is not expected that this process will be completed until the first week of next month.

Sources explained a pan-Scotland Day of Action is planned for February 7, three calendar months before polling day, when all the SNP's candidates will take part.

The party will hold its campaign conference in Glasgow on March 28/29.