Senior Scottish Conservatives fear Boris Johnson’s election as Tory leader could be a “catastrophe” for the UK, because it is likely to turbo-charge the campaign for Scottish independence.

Senior Tories believe Johnson’s zeal for Brexit will immediately boost support for a fresh referendum – a view echoed by Nicola Sturgeon, Scotland’s first minister, on Sunday. Johnson as prime minister would be “devastating, disastrous” for the party and make the case for independence stronger than ever, she said.

Scottish Tories fear Johnson’s election is a foregone conclusion given his sizeable lead among English Tory activists and members, a lead that could withstand the revelations about his late-night row with this girlfriend Carrie Symonds.

“He would have to be guilty of criminality on a huge scale to be stopped,” said one senior Conservative colleague and critic. “Our job is to move this from being a catastrophe to a challenge.”

Profile Boris Johnson's Tory leadership campaign Show Hide Personal style A late-night altercation between Tory leadership favourite, Boris Johnson, and his partner, Carrie Symonds initially changed the dynamics of Johnson's campaign. He had been either invisible or deliberately sober to the point of dullness, when his usual primary draw to Tory members is a self-created sense of optimism and fun. Much is also made of his supposed broad appeal to the electorate, evidenced by two terms as London mayor. His bizarre claim to make model cardboard buses has raised eyebrows. In most political contests, Johnson’s character – he has lost more than one job for lying, and has a complex and opaque personal life – would be a big issue, but among the Tory faithful he seemingly receives a free pass. It remains to be seen what impact that late-night police visit will have on his chances. Brexit He has promised to push for a new deal while insisting the UK will leave the EU come what may on 31 October, even if it involves no deal. While his hard Brexit supporters are adamant this is a cast-iron guarantee of leaving on that date, elsewhere Johnson has been somewhat less definitive. Asked about the date in a BBC TV debate, Johnson said only that it was 'eminently feasible', although he then went on to tell TalkRadio that the 31 October deadline was 'do or die'. Taxation His main pledge has been to raise the threshold for the 40% higher tax rate from £50,000 to £80,000, at a cost of almost £10bn a year, which would help about 3 million higher earners, a demographic with a fairly sizeable crossover into Tory members. Johnson’s camp insist it would be part of a wider – and so far unknown – package of tax changes. Public spending He has said relatively little, beyond promising a fairly small increase in schools funding, as well as talking about the need to roll out fast broadband across the country. Johnson has generally hinted he would loosen the purse strings, but given his prior fondness for big-ticket projects – London’s cancelled garden bridge, the mooted 'Boris island' airport – perhaps expect more of a focus on infrastructure projects than services. Climate and environment This is unlikely to be a big issue for Conservative party members, and Johnson has not said much on this beyond confirming his general support for the new government target of cutting greenhouse gas emissions to a net zero by 2050. Foreign policy Also unlikely to be a big issue among Tory members, beyond vague platitudes on 'global Britain', it could be a weak spot for Johnson given his poor performance as foreign secretary. He was seen as something of a joke by diplomats – both UK and foreign – and is likely to face more questioning over his gaffe about the jailed British-Iranian woman Nazanin Zaghari-Ratcliffe Peter Walker Political correspondent Photograph: Isabel Infantes/AFP

On Sunday, Sturgeon said Johnson was seen as one of the principal people responsible for the Brexit “mess” by those living in Scotland, where 62% voted remain in the 2016 EU referendum. She told Sky News’ Sophy Ridge on Sunday she was confident the yes campaign would win a fresh independence vote.

“He is seen in Scotland, I think, as one of the principal politicians who are responsible for the mess that we are in over Brexit, the guy who misled people in the EU referendum campaign and the guy who now says he is prepared to take the UK out of the EU without a deal. For most people in Scotland that is a horrifying prospect.”

A Panelbase poll in the Sunday Times Scotland this weekend found support for independence would jump to 53% if Johnson became prime minister, giving the yes campaign a six-point lead. It found he has a popularity rating among Scottish voters of -37, a worse rating than Nigel Farage.

During last week’s prime minister’s questions, Ian Blackford, the Scottish National party’s Westminster leader, used parliamentary privilege to call Johnson a racist. To cries of shame from Tory backbenchers, Blackford cited a satirical poem published in the Spectator while Johnson was editor, describing the Scots as a “verminous race”, his references to Africans with “watermelon smiles” and Muslim women wearing “letterbox” burqas. Sturgeon also said Johnson had made “overtly racist” comments.

Ruth Davidson, the Scottish Tory leaderwho is widely admired in the UK party, was highly critical of Johnson’s stance on Brexit during the 2016 referendum campaign, effectively calling him a liar on national television.

The Scottish party’s hopes are pinned on Jeremy Hunt, the foreign secretary. Most Scottish members are expected to vote for him, and Hunt flew to north-eastern Scotland on Sunday for an early campaign visit. He remains the rank outsider in the contest but the Panelbase poll suggests his victory could dampen support for independence, if only because it gave him a popularity rating of -24, 13 points better than Johnson’s.

Privately, many senior Scottish Tories are in despair. They have implored Johnson to disavow his previously intemperate language and invest his energies in preventing a surge in support for independence. “The big question mark is will Boris listen? I don’t know what the answer to that is,” said one former cabinet colleague.

Colin Clark, the Tory MP who unseated the former SNP leader Alex Salmond in the 2017 election, is a vocal supporter of Johnson – one of four Scottish Tory MPs who have backed him publicly. He believes his critics underestimate Johnson’s charisma and leadership skills – skills proven, Clark said, by twice winning elections to become mayor of London.

Clark took Johnson on a tour of Aberdeen recently, meeting voters. “They wanted to do a selfie with him; not one person was negative to him. As Etonian as he may be, he communicates with people,” Clark said.

Johnson, he added, was a “one nation Tory”, adding that the language Blackford quoted against him was a caricature, taken out of context. “I honestly believe Boris will bring a team together, including Scottish MPs, which will be better for the whole of the UK.”

Much hinges on whether Johnson reneges on his promises to some Tory backbenchers to force through a no-deal Brexit before 31 October. He faces immense pressure from senior Tories to reverse tack, including from the chancellor, Philip Hammond, and from industry and the City, due to the economic damage it would cause.

A recent YouGov poll of Tory party members underlined a growing gulf between the Scottish and English parties. It showed 63% would rather see Scotland independent than abandon Brexit – a position very few Scottish Tories share.

Johnson’s Scottish colleagues hope his habitual cynicism and opportunism will work in their favour and that once elected he will ignore his promises during the leadership campaign. “The practical thing is he only needs the members once [to get him elected],” said one.

Scottish Tories are already working on a strategy to soften the impact of a Johnson premiership. They want him to pledge his support for the union to lessen the damage of his perceived hostility to Scotland and its interests, as well as to float new concessions for Scotland, including a more flexible immigration system. Along with other parts of the UK dependent on migrant labour, this would allow Scotland to reverse its falling birth rate and protect the public services, universities and farms that need migrants. Theresa May’s £30,000 minimum salary for new entrants would be scrapped.

Scotland might also be offered new financial and tax powers, perhaps before the Scottish and UK governments start reviewing the fiscal framework set up after Holyrood was given near-complete control over income tax rates in Scotland.

Sturgeon recalled last week that after a recent second world war commemoration event, Johnson told her: “So Nicola, full fiscal autonomy. Does that buy you guys off?”

Blackford believes this crisis will come to a head quickly. “Because I don’t believe he can get no deal through, my best guess is he will be seriously tempted to go to the country, to go to the people with a no-deal Brexit,” he said.

If an election did take place, Sturgeon and the SNP would ask Scotland’s voters for a renewed mandate for independence. Faced with a weak and demoralised Labour opposition, recent opinion polls suggest they would get that mandate, comfortably.