Boris Johnson has said he is "not in the slightest" bothered by a rumoured plot by Scottish Conservative MPs to keep him out of Downing Street, dubbed "Operation A*se".

The former London mayor has been criticised in the past by Scottish Tory leader Ruth Davidson and Scottish secretary David Mundell amid reports of a Scots campaign to thwart his leadership ambitions.

Speaking at the only Tory leadership hustings in Scotland on Friday, Johnson and his fellow candidate Jeremy Hunt agreed in opposing a second Scottish independence referendum.

However, while Hunt vowed to "muster up my British politeness" and "say no" if Nicola Sturgeon asked for the power to hold a fresh independence vote, Johnson fell short of promising that.

The hustings event in Perth was chaired by STV's political editor Colin Mackay, who pressed each candidate in turn on their position on the Barnett formula - the system which allocates the Scottish Government funds from Whitehall.

Hunt said he would keep the formula for his entire premiership, should he succeed in his bid to become PM, but added: "I'm not someone who will say that every aspect of the Barnett formula works perfectly.

"But once you start to unpick something like that, it is a gift for Nicola Sturgeon and I don't think we should go there."

Johnson, appearing later, agreed: "I think the Barnett formula must stay. We must support our precious union, support it financially.

"I hope very much that the announcements I have made so far about funding education, about putting more money into police, will be reflected by the SNP government in how they handle that increase in the block grant."

Challenged by Mackay about previously describing the Barnett formula as a "present" from English taxpayers, he said: "You are talking about a comment I made 10 years ago."

Hunt, who is backed by Ruth Davidson, unequivocally ruled out consenting to a second independence referendum if he becomes Prime Minister.

"As prime minister of the UK I will never allow our union to be broken up," the foreign secretary said.

"And if the First Minister of Scotland asks me for a second independence referendum, I will muster up my British politeness and I will say 'no'."

Challenged on the same issue, Johnson said: "My view is that you had a referendum in 2014 when the people of this country were promised ... that this was a once-in-a-generation choice they were making."

Asked if he would accept a vote if there was a majority for it in Scotland, he answered: "I think there was a democratic vote by the people of Scotland which was pretty conclusive."

Pressed again on whether he was ruling out another referendum, he said: "I see absolutely no case for having a second referendum in Scotland. I think it's absurd."

Asked if he was bothered about so-called "Operation A*se,", Johnson replied: "Not in the slightest".

He said he had the support of "more than half the parliamentary party" at Westminster.

"So when it comes to the crucial work ahead of us of getting Brexit done, of building a coalition across our party, uniting everybody, I think we are most of the way there," he added.

Hunt also said if it happened, he would mitigate the impact of a no-deal Brexit to defend the integrity of the UK.

He said: "We should always be alive to the risks to the union but also confident about its great, great future."

"If we get things wrong then of course those risks increase."

Hunt added: "That's why, if we end up with a no-deal Brexit we have got to take every possible measure to prepare and support businesses in Scotland and other parts of the UK."

Johnson said a "successful, pragmatic Brexit could be a wonderful thing to entrench and intensify the union".