The SNP has complained to Sky News and Ofcom about the “deeply undemocratic” decision to exclude the party from a General Election TV debate.

Sky News proposed a live debate between Boris Johnson, Jeremy Corbyn and Jo Swinson, shortly after Ms Swinson said the Liberal Democrats would launch legal action against ITV if it did not include her in its proposed head-to-head debate between the Tory and Labour leaders.

The SNP said it has written to the head of Sky News calling for the party to be fully included in any debate, and making clear it could test the matter in the courts if it is not swiftly resolved.

SNP business convener Kirsten Oswald said: “The disgraceful decision to exclude the SNP from the Sky News debate is deeply undemocratic and completely inexcusable.

It’s time Sky News and other broadcasters stop short-changing voters and do their job by recognising the multi-nation, multi-party nature of UK politics

“This is another example of the Westminster establishment closing in on itself and showing no regard for Scotland.

“The SNP is the third party in the UK Parliament, the governing party in Scotland, and as past election results and current polling shows, the party most likely to retain this position and hold the balance of power after the election – there is no excuse for shutting us out and silencing Scotland.

“This election will be one of the most important in Scotland’s history. The result could determine the UK’s relationship with the EU and the course of Scotland’s relationship with the UK – making it all the more crucial that the SNP are fully included in the debates.”

Ms Oswald said polls show the “overwhelming majority” of voters in Scotland and across the UK agree that the SNP should be included in the debates.

“It’s time Sky News and other broadcasters stop short-changing voters and do their job by recognising the multi-nation, multi-party nature of UK politics,” she said.

We backed Sky News’s campaign for fair and representative TV debates, now we have the chance to have one, they look set to deliver the complete opposite

Meanwhile, a Plaid Cymru spokesman said the Sky debate, proposed for Thursday November 28, is one “harping back to a politics that no longer exists”, adding: “Every vote in Parliament is likely to count. Shutting out the third biggest party – the SNP – is lunacy, but in the last three years, Plaid Cymru’s votes have been an equally crucial factor on key issues in Parliament.

“To exclude us shows a lack of commitment to properly representing the parties which will decide the future of the UK.

“We backed Sky News’s campaign for fair and representative TV debates, now we have the chance to have one, they look set to deliver the complete opposite.”

On Monday, Ms Swinson said her party is taking legal advice and will “pursue legal avenues” if ITV does not change its format to include her.

When asked during her campaign launch on Tuesday what her message to Mr Johnson and Mr Corbyn would be regarding TV debates, Ms Swinson said: “My message is I hope to see you there, or if not: what are you afraid of?”

She added: “I think it is ridiculous to suggest that the country is well served by a debate between two men who want to leave the European Union.”

Ms Swinson said she “cannot wait” to take on the two leaders, and on the issue of being left out of the ITV debate, she said sexism “has to at least be one of the possible explanations”.