Boris Johnson has claimed Nicola Sturgeon and Jeremy Corbyn are "yoke-mates of destruction" over the threat they both pose to the future of the UK.
The Prime Minister hit the campaign trail in Scotland and accused the SNP and Labour leaders of cooking up a "shady" deal which would allow a referendum on independence to be staged in exchange for SNP MPs propping up a Corbyn administration which fails to win a majority on December 12.
Ms Sturgeon is to seek a Section 30 order before the end of the year from the new Prime Minister to allow Scotland to stage another vote on leaving the UK.
But Mr Johnson explicitly said today he will rejected such a request - even if the SNP wins a Holyrood majority in 2021 and dismissed Sturgeon's claims that Westminster's resistance will crumble if her party is returned with a majority north of the border on December 12.
The Tory leader, on a visit to the Maltings distilllery near Elgin, set out plans for a review of whisky duty.
Asked whether Ms Sturgeon or Mr Corbyn was a greater threat to the union, Mr Johnson declared: "They're yoke-mates of destruction."
And he insisted he would "absolutely" provide a "cast iron pledge" not to grant a section 30 order.
“There is no case whatever because people were promised in 2014 absolutely clearly that it would be a once in a generation event and I see no reason why we should go back on that."
Mr Corbyn has indicated that a second referendum could take place in the latter stages of a Labour administration if the people of Scotland want it. But Labour claims that no formal deal has been done with SNP - a claim derided by the Prime Minister.
"Pull the other one - it's got bells on, " he quipped.
"It's perfectly obvious that Jeremy Corbyn is going to rely on the SNP to get into power and to do that he's done a shady deal to have a second referendum."
He added: "The people of Scotland were told in 2014 that if they had a referendum that it would be a once in a generation thing. They were promised that I think by all parties on that occasion.I'm not surprised that there's a deal now between the SNP and Jeremy Corbyn.
"Obviously the only way that Jeremy Corbyn hopes he can get into power is with a deal with the SNP.
"The result is that next year will be a chaotic year of two referendums. One on Scotland when they were told in 2014 it would be a once in a generation thing. And one on the EU which would be an utterly bizarre because Corbyn has pledged to go off and renegotiate what is already an excellent deal and spend three months dong that.
"And then have six months in a referendum campaign when it's not even clear what his policy is or what he would do if people voted either leave or remain. The best thing for this country for this whole UK is to get Brexit done."