Previous attempts to get leaders to do a TV debate have failed
Plans for a series of televised leaders' debates in the run up to the next election will not include a place for the SNP, it has been announced.
Nationalist leader Alex Salmond said it was "unacceptable" to exclude the party that forms the government of Scotland.
But opponents accused Mr Salmond of "bully-boy" tactics.
Prime Minister Gordon Brown, Tory leader David Cameron and the Lib Dems' Nick Clegg agreed to election debates, on the BBC, ITV and Sky.
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.
THE LEADERS' DEBATES
First - ITV hosted by Alastair Stewart
Second - Sky hosted by Adam Boulton
Third - BBC hosted by David Dimbleby
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."
Mr Salmond added: "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."
Mr Salmond said he did not rule out legal action over the issue
But Labour's former Scotland Office minister David Cairns, the Inverclyde MP, 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."
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.
Meanwhile, Scottish Secretary Jim Murphy has 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."
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