Boris Johnson refused to give an explicit commitment to block a second independence referendum for Scotland if he becomes the next prime minister.
The former foreign secretary insisted he believed “passionately” in the union of four nations that make up the UK, and was clear he did not want an SNP government in Edinburgh to get the chance to re-run the 2014 poll.
But when asked if he would rule out such a vote, if First Minister Nicola Sturgeon wins a majority at the next Holyrood elections on a manifesto that includes a commitment to hold another constitutional ballot, Mr Johnson would only say that “we should stick to that promise” that the referendum would be a once-in-a-generation event.
Speaking on a visit to Glasgow, the Tory leadership hopeful said: “I’m a passionate unionist, I will defend it, fight for it with every fibre in my being, every breath in my body.”
Asked about the prospect of a second independence referendum – something Theresa May said she had “no hesitation” in refusing consent for – Mr Johnson said: “We had a referendum in 2014, I remember it well, I obviously campaigned on the side of the UK and the union because I believe in it passionately and I was delighted to see it win.
“It was a decisive win, and I don’t think there is any more need, speaking personally, to see any more referendums on that subject for this generation.
“We did it then, people voted then, they were assured that their votes were decisive, they were told this was a once-in-a-generation thing, and I think we should stick to that promise.”
Pressed on whether he would rule out another ballot, he repeated: “I think we should stick to that promise.”
Mr Johnson spoke out after touring the BAE Systems shipyard in Govan, where he saw HMS Glasgow, the first of the new Type 26 frigates being built for the Navy, being constructed.
He pledged he would return to Scotland “at the earliest possible opportunity” if he succeeds Mrs May and becomes the next prime minister.
And he also repeated his argument that a “good Brexit” could help to strengthen the links between Scotland and the rest of the UK.
Mr Johnson told reporters: “The union must come first, and I believe also as it happens, that a good, sensible, pragmatic Brexit will strengthen the union.
“If we come out of the EU in the way that we all hope and we get a good deal, and we are able to start doing free trade deals, not just with our EU friends but around the world, then what is the SNP, the breakaway party going to say then?
“Are they really going to want Scotland to join the euro, to join Schengen, to submit to the whole panoply of EU laws.
“And is it going to be the centrepiece of the SNP’s campaign that Scotland, having just got back control of its incredible fisheries, they should hand it over to Brussels? It doesn’t sound like a winning formula to me.”
He insisted: “I think that far from weakening the union a good sensible Brexit will spike the guns of the SNP and strengthen the union.”
Mr Johnson has already said he will give himself the title of Minister for the Union if he becomes PM.
“One of the things I have pledged is, I will be making sure we do everything in our power to intensify and cement the union,” he added.