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14 January 2015, 15:44 | Updated: 14 January 2015, 16:47
The case for the SNP being included in televised general election leaders' debates is "unanswerable'', the party has said, as it accused Labour of working with Ukip.
Ed Miliband, Nick Clegg and Nigel Farage have joined forces to increase the pressure on David Cameron to sign up for the debates.
The Labour, Liberal Democrat and Ukip leaders have sent letters to the Prime Minister warning that it would be ''unacceptable'' for him to block the clashes.
They are demanding Mr Cameron - who last week made clear he would not participate unless the Green Party was included alongside Ukip - be ''empty chaired'' by broadcasters if he does not give ground.
The SNP said the parties' plea to the Prime Minister called for a format which would exclude the SNP, Plaid Cymru and the Greens.
SNP Deputy Leader Stewart Hosie said: "Ukip is now part of the Westminster establishment along with Labour and the Lib Dems, and this proves it.
"It's quite astounding that Labour are prepared to work with Ukip on a debate format which excludes the SNP, Plaid Cymru and Greens. Yesterday Labour voted with the Tories on austerity, today they are working with Ukip on debates.''
He said recent polling indicated that most people across the UK support First Minister Nicola Sturgeon being included in the debates.
"With a larger membership than the Lib Dems and Ukip combined, and more elected MPs than Ukip, the case for including the SNP is unanswerable,'' Mr Hosie added.
"The addition of the SNP, Plaid and the Greens with Nicola Sturgeon, Leanne Wood and Natalie Bennett will also rightly show that politics across the UK isn't just an old boys club.''
Broadcasting regulator Ofcom said it was minded to class Ukip, but not the Greens, as a "major party'' in a report published earlier this month.
The inclusion does not directly affect which leaders are invited to join the televised debates, which is a matter for broadcasters, but the regulator's rules require that "due weight'' is given to each party during election coverage.
The televised debates proposed include Mr Farage appearing alongside Mr Cameron, Mr Miliband and Mr Clegg on an ITV programme.
The letters to Mr Cameron state: "''I hope you will agree that the decision as to who should take part in the televised debates should not be in the hands of any party leader, each of whom inevitably has their own political interests to defend. It must be a decision independently and objectively arrived at.
''It would be unacceptable if the political self-interest of one party leader were to deny the public the opportunity to see their leaders debate in public.
''Therefore, if you are unwilling to reconsider, the three party leaders who have committed to participate will ask the broadcasters to press ahead with the debates and provide an empty podium should you have a last-minute change of heart.``
Mr Cameron was also challenged on the subject at Prime Minister's Questions at Westminster.
Responding to accusations from Mr Miliband that was "running scared'', he said: ''I'm all for these debates but you cannot have two minor parties without the third minor party. Why is he frightened of debating the Green Party?''
He said Ukip and the Greens had both beaten the Lib Dems in recent European elections. ''You either have both of them, or you have none of them.``
Speaking on a visit to Aberdeenshire, Ms Sturgeon said: "It's a simple democratic argument. The SNP is Scotland's biggest political party, we're also the UK's third biggest political party.
"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.
"There's no argument against having the SNP in these debates and if we weren't included then it would be an absolute democratic outrage.''