The SNP is to take legal action against ITV over its exclusion from the broadcaster's general election debate.
ITV plans to show a head-to-head debate between Conservative leader Boris Johnson and his Labour counterpart Jeremy Corbyn next week.
But SNP leader Nicola Sturgeon said it was "fundamentally unfair" to not include her party, which is the third-largest in the UK.
The Lib Dems have already launched a legal challenge to ITV's plan.
Ms Sturgeon said the decision to only invite Mr Johnson and Mr Corbyn to take part in the televised debate on 19 November "fails to recognise that the UK is no longer a two party state".
She added: "The SNP is the third party in terms of Commons representation in the last Parliament, we are the governing party of Scotland and we are one of the biggest political parties in the whole of the UK in terms of membership.
"It is also entirely possible that we will hold the balance of power in the House of Commons after this election - making it all the more important that our perspective is heard and indeed scrutinised.
"To exclude the SNP would be a fundamental breach of broadcasters' obligations to fully and properly represent and reflect the views of the whole UK."
Ms Sturgeon said her party would be launching a fundraiser to support the costs of the legal challenge, which she wants to be heard on Monday alongside the one from the Liberal Democrats.
But she said that, unlike the Lib Dems, the SNP would be "arguing not just for the SNP but for other parties to have a place in this debate as well, just as was the case in the ITV leaders' debate of 2015".
When ITV announced its plans, the channel said it would hold a live interview-based programme alongside the leaders' head-to-head to allow other parties to comment.
It also said it would host another multi-party debate ahead of the election on 12 December.
The BBC will host a live head-to-head debate between Mr Johnson and Mr Corbyn in Southampton on 6 December, and will also broadcast a seven-way podium debate between senior figures from the UK's major political parties on 29 November, live from Cardiff.
And BBC Scotland will stage a televised debate between the SNP, Conservatives, Labour and Liberal Democrats on 10 December, although the Scottish Greens have criticised the decision not to include them.