Scottish independence: Boris Johnson says he will never agree to indyref2

It comes after Ms Sturgeon insisted another poll would be "irresistible" if the SNP wins the majority of Scottish seats at the general election on December 12.

Camley's Cartoon: Boris Johnson will never agree to second independence referendum.

Mr Johnson made his latest comments during a fleeting campaign visit to Scotland, where he toured the Roseisle distillery near Elgin before flying to Northern Ireland.

He pledged to review the amount of tax paid on a bottle of whisky under a Conservative government, in a move that will be welcomed by an industry recently hit with US export tariffs of 25 per cent.

However, the Prime Minister was repeatedly questioned over his apparent reluctance to meet voters in Scotland, with his last three visits largely taking place behind closed doors.

Asked if he would grant a second independence referendum, Mr Johnson said 2014's vote was supposed to be a "once in a generation thing". 

He said a deal between the SNP and Labour would see a "chaotic year of two referendums" – one on independence and one on EU membership. 

He said: "It’s perfectly obvious that Jeremy Corbyn is going to rely on the SNP to get him into power and to do that he’s done a shady deal to have a second referendum."

Labour has ruled out any deals or pacts with the SNP if Mr Corbyn fails to secure a majority, while the SNP has rejected any formal coalition.

Asked if he could give a cast-iron pledge to Scottish voters not to agree a second referendum, Mr Johnson said: "Absolutely. 

"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 pledge."

He later said it would make no difference if the SNP wins a pro-independence majority in the 2021 Scottish Parliament elections, and branded Mr Corbyn and Ms Sturgeon "yoke-mates of destruction".

It came as the Referendums (Scotland) Bill, which provides the framework for any future independence vote, passed its first stage in Holyrood by 65 votes to 55.

Elsewhere, the Prime Minister said it was "absolutely untrue" to suggest he was frightened of Scottish voters. 

He added: "I've met many, many Scottish voters in the course of my time as Prime Minister and I look forward to meeting many more."

Mr Johnson said he had met voters in Scotland "loads of times", but failed to give examples of where and when.

He then turned to a journalist and added: "You're a voter, aren't you?"

Asked when he would hit the streets in Scotland, he continued: "I've met many, many members of the public. You haven't necessarily been there."

Mr Johnson confirmed a Conservative government would review alcohol duty in a bid to provide better support for domestic drinks producers. 

The Tories said this would help boost the £5.5 billion Scotch whisky industry, which supports 42,000 jobs across the UK. 

The Prime Minister told reporters he has "interceded several times" with Donald Trump to try to get new US whisky tariffs dropped.

He said: "You know why this happened, why they put a tariff on Scotch whisky?

"It's because the EU Commission decided to put a tariff on bourbon so the Americans automatically retaliated by hitting whisky.

"It was cynically triggered by the EU Commission because they knew that the Americans will respond in that way.

"Once we come out of the EU, once we get Brexit done, those tariffs will no longer apply to this country.

"But we're hoping to get rid of them even sooner than that, and I've certainly asked the president to lift them."

Karen Betts, chief executive of the Scotch Whisky Association, welcomed the announcement of a review. 

She said: “Reform of alcohol duty has been a long term goal of the Scotch Whisky industry and is our number one ask of all political parties in the General Election. 

“This announcement is welcome, and opens the door to reforming a broken system in which large inconsistencies between alcohol categories put Scotch whisky and the wider UK spirits industry at a competitive disadvantage.

“A simplified alcohol duty regime in the UK to better reflect alcohol content would be fairer for consumers, increase competitiveness and, according to robust, independent economic research, remain an important driver of tax revenue to support public services.

“Of course, the devil is in the detail and we look forward to working with HM Treasury officials on our detailed proposals submitted ahead of the Budget to ensure we have an alcohol tax system fit for the 21st century.”

Douglas Ross, the Scottish Conservative candidate for Moray, said it was "great news for Scotland's whisky industry".

He added: “It was the whisky industry's key ask for this election campaign and, thanks to the Prime Minister’s announcement today, a future UK Conservative government has now pledged to roll out action immediately."

Mr Johnson's visit to Scotland followed a rocky start to the Conservative Party's election campaign. 

Its first few days have been mired in controversy after Jacob Rees-Mogg suggested the Grenfell fire victims lacked common sense and Alun Cairns quit as Welsh Secretary over a former aide's role in a collapsed rape trial.

Meanwhile, Aberdeen South MP Ross Thomson, who led Mr Johnson's Conservative Party leadership campaign in Scotland, announced he would not stand for reelection after being accused of sexually assaulting a Labour parliamentarian. 

Paul Sweeney, the Labour MP for Glasgow North East since 2017, claimed Mr Thomson groped him in the Strangers' Bar in the Commons last year.

Mr Thomson has denied the allegations, insisting they are "false and defamatory" and a "political smear". 

The Prime Minister dodged a question about whether Mr Rees-Mogg should resign, saying that the Leader of the House of Commons had apologised for his remarks.

However he said Mr Thomson, a staunch Brexit supporter who was seen as a close ally in Scotland, was right to step down. 

He said: “I think that Ross has obviously taken the right decision and I think that you should direct all further questions to him. 

"And obviously I regret very much what has happened, but he has done the right thing.”

He added: “I think Ross has done the right thing.”

Speaking ahead of Mr Johnson's visit, Ms Sturgeon said he should "apologise for the chaos he and his party have subjected us to for years".

She added: “He is a prime architect of the Brexit vote and the utter shambles it has now led to."

Scottish Liberal Democrat leader Willie Rennie accused Mr Johnson of "undermining the integrity of the United Kingdom by backing a deal that puts a border down the centre of the Irish sea".

He said: “Voters in Scotland cannot trust the Conservatives. They have put Brexit before the union."