The Scottish Conservatives would boycott any “wildcat” referendum on independence from the UK, the party’s leader declared on Monday amid a renewed push by the governing Scottish National party to end the three century-old union with England.

The SNP announced at the weekend that it would seek to hold another independence referendum after May elections for the Scottish parliament — without UK government approval if necessary.

Voters in Scotland rejected independence by 55-45 per cent in 2014, but opinion polls over the past year suggest another referendum on the issue would yield a majority for leaving the UK.

At an online event, Douglas Ross, Scottish Tory leader, said any new plebiscite must follow the “gold standard” of the 2014 vote, which was approved by the UK government under devolution legislation.

Boris Johnson’s spokesperson reiterated on Monday that he would not approve another referendum. “The UK is the most successful political union the world has seen. The PM has always said that the Scottish people have had a vote on this issue and decided to remain part of the UK,” the official said.

The UK prime minister is expected to visit Scotland later this week, when he will again make the case for the union.

Mr Ross said the priority for Scottish politicians should now be on improving the economy and education rather than the constitution.

“That’s where the focus should be, not on wildcat referendums, which I would absolutely boycott because they would be an absolute waste of precious time and resources,” he told an online event organised by the Centre for Policy Studies think-tank.

The UK government insists any unapproved independence referendum would not be legal, but the Scottish government has argued that an advisory vote could be held under current devolution legislation.

The court of session in Edinburgh last week heard arguments in a case brought by a pro-independence activist that backers hope will yield a definitive clarification of the law.

Under the new SNP plan — unveiled at the weekend following pressure from members for a “Plan B” to deal with continued Westminster refusal of a referendum — the party said it would seek to pass legislation preparing for a vote if there is another pro-independence majority following Scottish parliamentary elections scheduled for May 6.

The SNP said Westminster would have to decide whether to approve the vote or “take legal action to dispute the legal basis of the referendum and seek to block the will of the Scottish people in the courts”.

Support for the SNP and independence has been buoyed by widespread disaffection at the UK’s exit from the EU and by perceptions that Ms Sturgeon has handled the coronavirus epidemic better than Mr Johnson.

However, while polls point to a landslide SNP victory in May, the party has suffered growing divisions in recent months and an extraordinary feud between Ms Sturgeon and her predecessor as first minister Alex Salmond.

Mr Salmond has accused his former protégé of misleading parliament about her government’s handling of complaints of harassment against him, a charge she denies. Ms Sturgeon has also dismissed suggestions from Mr Salmond’s supporters that her allies conspired against him.

Ms Sturgeon’s husband, the SNP chief executive Peter Murrell, has faced scrutiny from a Scottish parliamentary committee looking into the handling of the complaints against Mr Salmond.

Mr Murrell told the committee last month he regretted the wording of a 2019 text message to a party colleague in which he suggested it would be good to be “pressurising” police over the accusations against Mr Salmond.

On Monday, Scottish Labour interim leader Jackie Baillie wrote to prosecutors calling for “an immediate investigation” into whether Mr Murrell perjured himself when he told the committee that there were no other messages on the issue beyond the two from him already made public.

Ms Baillie, a member of the committee, said it had since come to its attention that other messages were being held by prosecutors.

The SNP did not immediately respond to a request for comment on Ms Baillie’s letter. Mr Murrell could not be reached for comment.

Under legal challenge from Mr Salmond, the Scottish government in 2019 accepted that its investigation into harassment complaints against him was “was tainted by apparent bias”. Mr Salmond was last March acquitted at the High Court in Edinburgh of all 13 sexual offence charges against him.