NICOLA Sturgeon has mocked Boris Johnson over his insistence that he will never face her in a televised head-to-head debate nor facilitate a second independence referendum as long as he is in Downing St.

The First Minister took to social media to give her response, tweeting: “Is this the ‘die in a ditch’ definition of ‘never’?” She added a laughing emoji.

Her colleague, Ian Blackford, who leads the SNP at Westminster, accused the Prime Minister of "running scared".

In an interview with The Herald Mr Johnson, asked if he would ever engage the SNP leader in an election debate, replied: “Once Nicola Sturgeon takes leadership of her party in Parliament[ie Westminster] and is a serious contender to be Prime Minister of the UK that would be the appropriate moment.

READ MORE: Boris Johnson: I'll never debate Sturgeon and I'll never agree to a second Scottish independence referendum

When it was suggested that meant he would never debate the SNP leader, the PM replied: “The appropriate thing in this election is for me to contest the debates with whoever theoretically could be Prime Minister.”

Earlier this month at a rally in Glasgow, Ms Sturgeon asked why the PM and Mr Corbyn were “scared to debate me,” and issued a challenge, saying: “I’ll debate you anytime and anywhere and it will be for you to justify why Scotland can’t have a choice on our own future.”

Mr Johnson accepted the SNP leader could “very influential in our politics” but pointed out she had no seat in the Commons nor could she be PM.

“The candidate to be Prime Minister who Nicola Sturgeon would support is Jeremy Corbyn and that is why he is the appropriate person[to debate].”

Asked if he thought the FM was beneath him, Mr Johnson replied: “No, of course not. All I am saying is there is only one[other] person who can be Prime Minister and that is Jeremy Corbyn.”

The PM’s decision means there will be no opportunity for Mr Johnson and Ms Sturgeon to confront each other in a debate during the election campaign. A seven-way debate is being planned by the BBC but this is billed as involving party leaders or senior party figures. It is thought likely that Michael Gove, the Cabinet Office Minister, will represent the Conservatives.

Mr Blackford Ian Blackford said: "Given that every intervention Boris Johnson has tried to make in Scotland has backfired spectacularly for him, it's no wonder that he is running scared of Nicola Sturgeon.

READ MORE: Scottish Secretary: SNP majority in 2021 is 'democratic mandate' for Indyref2

"The Prime Minister's latest threat to ignore people in Scotland only serves to remind voters how supremely arrogant and out of touch he is and boosts the case for voting SNP.

"If Mr Johnson is so confident in his completely undemocratic position, why is he afraid to debate it publicly?" asked the party leader. 

“Even Johnson’s Scottish Secretary immediately undermined his position by conceding that if the SNP get a majority in the 2021 Holyrood elections, that would be a mandate for a referendum. The reality is that the Tories’ opposition to an independence referendum simply isn’t sustainable and is crumbling," declared Mr Blackford.

"The SNP is the main challenger in every Tory-held seat in Scotland, and at the crucial election on 12th December, Scotland can send the strongest message possible by voting SNP to boot the Tories out of government, escape Brexit and put Scotland's future in Scotland's hands not Boris Johnson's," he added.

Yesterday, the SNP and Liberal Democrats lost a court bid to get their leaders onto tonight’s ITV debate between the PM and Jeremy Corbyn.

The hour-long programme, starting at 8pm, will be followed at 10pm on ITV with interviews, involving those leaders who did not take part in the debate ie Ms Sturgeon, Jo Swinson for the Lib Dems, Nigel Farage for the Brexit Party and Sian Berry for the Greens.

Both Mr Johnson and Mr Corbyn will spend today preparing for tonight’s live debate, which could prove to be a significant moment in the election campaign.

It is thought that in such head-to-heads the incumbent has more to lose because he or she has a record to defend.

However, the Tory leader sought to get in his attack early, writing to his Labour counterpart demanding to know if he would back Leave or Remain in a second referendum, if immigration would be higher or lower under a Labour Government and how much Labour would pay for market access to the EU.

An opinion poll is being planned to give the public’s snap response to the debate.