The SNP has launched a legal challenge against ITV's decision to exclude other political parties from a general election leaders' debate between Boris Johnson and Jeremy Corbyn.
Nicola Sturgeon, said it was "unacceptable" other parties would be "cut out" of the televised debate.
She also criticised the Liberal Democrats for taking similar action, but only on behalf of Jo Swinson, and as a result the SNP wants its action for all leaders to be able to participate to be heard in court on Monday "in parallel" with the Liberal Democrats' case.
The party has also launched a crowdfunder to pay for the costs of the legal challenge,
Ms Sturgeon said: "We are asking that this legal challenge be heard on Monday in parallel with that taken by the Liberal Democrats - but unlike their self-centred approach, we will 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.
"The era of two-party dominance of UK elections is fading fast, and it is incumbent on broadcasters to recognise that and to act accordingly."
As it stands, the ITV programme would be a head-to-head between the Conservative and Labour leaders on November 19.
But Ms Sturgeon said: "This election may be the most important of our lifetimes. It is quite simply unacceptable for the SNP and other parties to be cut out of proposed leaders' debates by broadcasters. This is fundamentally unfair and it fails to recognise that the UK is no longer a two party state.
“And in Scotland, this would not be a debate between the two leading parties - instead it would ignore the country’s largest party, which according to the latest polls is also on track to win the third largest number of seats UK wide. By contrast, the latest polls show Labour in fourth place in Scotland."
When ITV announced the debate, it said it would hold a live interview-based programme alongside the leaders' head-to-head to allow other parties to comment, as well as another multi-party debate ahead of the 12 December poll.
But Ms Sturgeon said this was not adequate. “Crucially by the time the second debate with all party representation takes place, many voters will already have cast their postal ballots, meaning this debate gives Labour and the Tories an inbuilt and unfair advantage.
"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."
And she launched a crowdfunder to pay for the action. “For those who ensure that the SNP’s voice is heard and tackle this fundamental unfairness – we will be launching a fundraiser to support the costs of taking this case and securing fair representation."
The SNP, which boasts the largest membership of any party, reported a surplus of £1,119,353 last year.