Boris Johnson has made a “cast iron” pledge that he will not grant Nicola Sturgeon the powers she needs to hold a second independence referendum, regardless of whether the Scottish National party (SNP) wins a majority of Scottish seats in December’s general election or if they win a pro-independence majority in the Holyrood elections of 2021.

In his strongest rebuff yet to Sturgeon’s vow to hold a second vote on independence next year, Johnson used his first visit to Scotland of the election campaign to insist: “Absolutely, there is no case whatsoever [for a second referendum] because people were promised in 2014 that it would be a once-in-a-generation event and I see no reason why we should go back on that pledge.”

Play Video 1:55 'Stop running scared,' Nicola Sturgeon tells Johnson and Corbyn – video

Describing Nicola Sturgeon and Jeremy Corbyn as “yoke-mates of destruction” in terms of the future stability of the union, he told reporters: “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.”

On Wednesday, the leader of Scottish Labour, Richard Leonard, categorically ruled out any form of electoral deal with the SNP, after Johnson warned against making 2020 the “year of two referendums” and Sturgeon told voters that demand for a second vote on independence would become “irresistible” if her party were to win the election in Scotland.

Johnson’s visit comes on the second day of the election campaign, which has already been overshadowed for the Conservatives by a series of damaging blunders, after his ally, Jacob Rees-Mogg, suggested that victims of the Grenfell Tower fire lacked common sense and Alun Cairns, the Welsh secretary, resigned over his knowledge of a former aide’s role in allegedly sabotaging a rape trial.

Johnson visited the Roseisle distillery, near Elgin, in Moray, accompanied by the local Scottish Conservative candidate, Douglas Ross, who won the seat from the SNP’s former Westminster leader Angus Robertson in 2017.

Johnson spent little over an hour touring the rural distillery in a strictly managed visit during which he had no contact with the public. He returned immediately to RAF Lossiemouth, and from there travelled to Northern Ireland where he visited a Tayto crisp factory in Tandragee, County Armagh, to round off his day of campaigning.

Facebook Twitter Pinterest Boris Johnson at the Tayto crisp factory in County Armagh. Photograph: Daniel Leal-Olivas/Pool/AFP via Getty Images

Johnson was asked why his visits to Scotland since his election to the Tory leadership had all taken place in secure locations away from the general public, such as the Faslane nuclear base.

He insisted: “I’ve met many, many Scottish voters in the course of my time as prime minister and look forward to meeting many more.”

Pressed on where and when these meetings had occurred, he repeated that he had met Scottish voters “loads of times”. When the Guardian suggested that this was not the case, Johnson replied: “You’re a voter, aren’t you?”

Asked about his first meeting with Nicola Sturgeon after he became prime minster, when he was booed by crowds outside the first minister’s official residence, Bute House in Edinburgh, he said: “I’m always very pleased to come to Scotland.”

Johnson also claimed he had overruled advice in order to hold the COP26 climate change summit in Glasgow next year. “I overruled advice from others and I decided to put the COP26 climate change summit not in London … but in Glasgow, because it’s a fantastic place and it gives us the chance to show off Glasgow to the rest of the world.”

