The broadcaster is hosting a head-to-head between Prime Minister Boris Johnson and Labour leader Jeremy Corbyn on November 19, but has not invited Ms Swinson or SNP leader Nicola Sturgeon to take part.
Lib Dem president Sal Brinton said today the decision "disrespects" those who voted Remain in the 2016 Brexit referendum. Ms Swinson, the UK party leader, has previously said it was wrong to exclude "a voice of Remain" and the only woman who could be the next prime minister.
But the SNP said it was "outrageous" for a broadcaster to even consider including the Lib Dems - who had just 12 MPs before parliament was dissolved for the December election - and not the Nationalists, who were the third largest party in the Commons.
The SNP is now considering its next steps over its exclusion from the prime-time debates. The Nationalists previously launched a judicial review in 2010 in an attempt to have Alex Salmond included in the leaders' debates broadcast ahead of the 2010 general election, only to be told by a judge they had left the legal challenge too late.
SNP business convener Kirsten Oswald said: “It is utterly outrageous for any broadcaster to include a minor party like the Liberal Democrats and to exclude the SNP – who secured nearly three times as many seats at the last election, and whose current support suggests will continue to be the third largest party in Westminster.
"We have made that clear to Sky, whose proposed debate is a democratic disgrace, and to ITV - and we are considering our next steps. In all of this, one thing that is clear, is the utter hypocrisy of the Lib Dems who are happy to see a larger female led party of remain excluded in their own self-interest as part of a typical Westminster stitch-up.”