Mr Johnson, who declared he was “pumped” and “coiled like a spring” for this evening’s debate, said he would only share a stage with the First Minister if she led her party at Westminster and was a candidate to be prime minister.

The SNP claimed the two main party leaders were “scared” of Ms Sturgeon and called on them to commit to take part in a seven-party debate in two weeks.

Lib Dems and SNP lose legal challenge against ITV televised election debate excl...

The SNP claimed the two main party leaders were scared of Ms Sturgeon. Pictures: PA

Although she can be very influential in our politics, she has no seat in the House of Commons, nor can she be prime minister.

“The candidate to be prime minister, and the candidate who Nicola Sturgeon would support, is Jeremy Corbyn. And that’s why he’s the appropriate person to debate.”

Asked if he was “brave” enough to take on the SNP leader, Mr Johnson added: “Once Nicola Sturgeon takes leadership of a party in Parliament and is a serious contender to be prime minister of the UK, that would be the appropriate moment.”

The SNP and Lib Dems lost a High Court challenge against ITV over its decision to exclude their party leaders from tonight’s TV debate. Lawyers acting for the parties argued that

the programme was unlawful because it breached broadcast impartiality rules.

But two leading judges ruled the decision was not open to challenge in the courts and that the parties’ only recourse was to complain to Ofcom.

1 December

ITV is due to host a seven-party debate on 1 December, although Mr Johnson and Mr Corbyn have not committed to taking part.

SNP Westminster leader Ian Blackford said following the judgment that Ms Sturgeon was “ready, willing and able” to take on the two main party leaders.

“Now it is incumbent on them to commit to the all party debate on 1 December or tell everyone what they are so scared of,” Mr Blackford said. “It was already clear that the Westminster political system is utterly broken and incapable of properly representing Scotland’s interests.

“What is now clear is that the UK broadcasting system is similarly incapable. Indeed the result of the decision to exclude the SNP is to discriminate against Scottish voters and to effectively treat them as second-class citizens. That is, quite simply, a democratic disgrace, and the fact that election law and broadcasting codes allow such gross unfairness is unacceptable.”

Lib Dem party president Sal Brinton said it was an “incredibly disappointing verdict”.

“It is worrying that the Ofcom guidance allows TV executives, not the voters, to decide whether the biggest issues of the day are debated openly in the ITV debate,” Baroness Brinton said.

“This campaign is undeniably dominated by Brexit, the single biggest issue for our country, perhaps in the last 75 years. Both Boris Johnson and Jeremy Corbyn are determined to drag this country down in the name of Brexit.

“This law needs to change.”

Lord Justice Davis, sitting with Mr Justice Warby, ruled that, even if the court did have jurisdiction to deal with the case, the format of the debates was a matter of “editorial judgment” and there was “no arguable breach of the Broadcasting Code”.

He added: “The clear conclusion of this court is that, viewed overall, these claims are not realistically arguable.

ITV lawyers told the court that the debate – and an interview with Lib Dem leader Jo Swinson due to follow it – would have been pulled from its schedule altogether if the judges had found ITV breached its duties under the code.

Lawyers for the Lib Dems had argued that Brexit is the “dominant” issue of the general election and that “the voice of Remain has been excluded”.

The SNP’s lawyers said the Nationalists represent views which would not be aired in a debate between Labour and the Conservatives, including on Brexit and Scottish independence.

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

Boris Johnson: I will never do a TV debate with Nicola Sturgeon