It met with an angry reaction from party leader Alex Salmond who said a legal challenge to the proposed debates was a possibility.

Opponents accused Mr Salmond of “bully-boy” tactics and said he was not even standing at the Westminster election next year.

But the First Minister said: “”It is entirely unacceptable to Scotland as well as to the SNP for the broadcasters to exclude the party that forms the government of Scotland - and indeed is now leading in Westminster election polls.

“If these debates are to be at all relevant to their audiences, they must reflect the democratic reality of Scotland and political diversity across the UK.

“And that must include SNP involvement in debates broadcast in Scotland.”

Prime Minister Gordon Brown, Tory leader David Cameron and Nick Clegg of the Lib Dems today agreed to terms for a series of the debates, BBC, ITV and Sky to host one each in the campaign.

It is the first time in British political history that the main party leaders have agreed to take part in a televised debate.

Both the BBC and Sky announced plans for separate debates in Scotland, while ITV said it would observe its obligations of due impartiality in its electoral coverage.

But Mr Salmond said the SNP was ahead in the latest Scottish opinion polling for Westminster and he did not rule out a legal challenge.

He told BBC Radio Scotland: “We are loath to take legal action.

“But in terms of defending the Scottish interest and defending the rights of access to democracy, then obviously you have to explore every possible avenue.”

But Labour’s former Scotland Office minister David Cairns, the Inverclyde MP, today dismissed Mr Salmond’s position.

He said: “The SNP’s thinly-veiled but sinister threat to ban Scottish viewers from watching debates between the people vying to become Prime Minister is a bully-boy tactic unbecoming of a democratic party.

“Alex Salmond is looking increasingly ridiculous.

“He is desperate to appear in a Westminster TV debate when he isn’t even a candidate for Westminster.”

The first debate will be produced by ITV with presenter Alastair Stewart; Sky will produce the second presented by Adam Boulton; and David Dimbleby will present the third for the BBC.

Scottish secretary Jim Murphy today called for a Scottish version of the debates in addition to the UK versions, and has asked his SNP, Lib Dem and Tory counterparts to take part.

Mr Murphy said: “I have asked for their support for this debate to be broadcast by BBC Scotland, STV and Sky, and for discussions to agree the format and ground rules.”