The SNP have written to the BBC, Sky and ITV informing them that they would be “unhappy” if the SNP had no representation in what will be the first American-style televised election debates in British political history.

Prime Minister Gordon Brown yesterday signalled his support for some form of TV debate.

A spokeswoman for Alex Salmond said the party remained “flexible” on the precise format, but said they could not rule out taking legal action that would block a debate restricted to Brown, Cameron and Clegg from being broadcast in Scotland.

“We would argue that any of the three debates that have been proposed … should not be aired in Scotland if there was no representation of the party that forms the current Scottish government,” said the spokeswoman.

Writing in the Sunday Herald today, right, Salmond argues the SNP’s voice should not be silenced by “rigged” debates.

The SNP say they have been given assurances that there is no technical barrier preventing Sky, the BBC and ITV from limiting the political debates to England, Wales and Northern Ireland.

The SNP’s determination to ensure Salmond is not bypassed in the prime-time debates – even though he could not become UK prime minister and whose party only fields candidates in Scottish constituencies – comes as Gordon Brown finally agreed “in principle” to take part in televised debates.

However, the statement from Number 10 yesterday appeared to suggest that the prime minister is still trying control the format of the debates. In it he hinted at separate one-on-one debates with the Conservative leader, David Cameron, and the Liberal Democrat leader, Nick Clegg.

It was also suggested by Brown that because of Britain’s cabinet system of “collective responsibility” he would like contests between key members of his cabinet and their shadow opposite numbers.

Clegg said he welcomed the decision to debate by the prime minister and said he was not concerned that a Brown-Cameron only contest had been floated.

The SNP’s legal challenge, if the BBC, Sky and ITV push ahead with only the three main party leaders up against each other, will break new electoral and legal territory.

The Representation of the People Act ensures broadcasters give fair time to all parties contesting an election. “Time” for each party is calculated over the course of a general election campaign and covers the amount of party political broadcasts and also ensures mainstream political programmes devote adequate time to all parties.

If three Brown-Cameron-Clegg debates – on the economy, on domestic policy and on foreign affairs – were broadcast in Scotland and did not include any representation from the SNP, it could be argued that one of Scotland’s main parties was being illegally excluded from the debate.

If the SNP were refused participation in all three debates but their case for representation was upheld in Scottish court, UK broadcasters could face further legal battles.

Deciding not to broadcast the debates in Scotland could leave Sky, ITV and the BBC – who recently united to offer the debates – open to injunctions from a wide range of political institutions who could complain that Scotland was being marginalised in a set of historic debates.

Last night the SNP’s election co-ordinator, Stewart Hosie, said the next general election in Scotland would be a “two-horse race between Labour and the SNP, so it would be entirely unacceptable, and unfair to the Scottish audience, to exclude [from the debates] the party that forms the government of Scotland”.

He added: “Broadcasters must meet their obligations to audiences across the UK – and that means that debates broadcast in Scotland must include Alex Salmond.”

A BBC spokesperson said last night that the broadcaster could not comment on Alex Salmond’s demands, but added: “Each of the broadcasters will seek to make suitable arrangements for ensuring due impartiality across the UK.”

Similarly, a spokesperson for ITV said: “It is the intention of the broadcaster to work together and discuss with the political parties how to ensure due impartiality throughout the UK.”

In a statement, Sky said: “The broadcasters have formed a joint team to discuss detailed plans with the political parties. Each of the broadcasters will seek to make suitable arrangements for ensuring due impartiality across the UK.”

However, on Friday Sky’s political editor, Adam Boulton, said: “There are genuine concerns about making sure the other parties are represented – the Greens, SNP, BNP, UKIP.”