The SNP is taking legal action against ITV after being excluded from a General Election leaders' debate.

Nicola Sturgeon confirmed the action after the broadcaster said it would host Boris Johnson and Jeremy Corbyn in a head-to-head on November 19.

Liberal Democrat leader Jo Swinson earlier this week said her party will launch a legal challenge over the decision to exclude all parties other than the Conservatives and Labour from the debate.

READ MORE: Decision to omit SNP from Sky election debate 'defies democracy' says Nicola Sturgeon

Scottish First Minister Ms Sturgeon said it is "fundamentally unfair" for other parties to be cut out of the debate.

She 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."

Sky News has proposed a November 28 date for its debate, while the BBC has confirmed it will host two debates, on November 29 and December 6, in addition to a series of Question Time specials.

Ms Sturgeon claimed the timing of debates following the initial ITV contest means many votes will already have been cast.

She said: "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.

"Including all parties would also give the public what they say they want, with polls showing 53 per cent of voters across the UK, and 73 per cent in Scotland, want to see that approach taken.

READ MORE: Nicola Sturgeon blasts SNP exclusion from TV leaders debate

"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.

"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."