Scotland’s governing Scottish National party has announced plans to try to hold a second referendum on leaving the UK — without Westminster approval if necessary — if there is a pro-independence majority in the parliament in Edinburgh after elections due in May.

It remains uncertain whether an SNP government really could hold such a vote. Meanwhile, pro-union politicians are still weighing how to stop the sundering of the UK.

The SNP said at the weekend that if Boris Johnson continues to refuse to authorise a rerun of the 2014 referendum, in which Scots backed staying in the UK by 55-45 per cent, it would seek to legislate for an advisory vote under current devolution law and challenge the UK prime minister to try to block it in the courts.

The UK government insists that such a referendum would be illegal without its approval, which Mr Johnson has said should not be granted until at least 2050.

A court case brought by a pro-independence activist is seeking a judgment on whether the Scottish parliament has powers to hold a referendum. But the Scottish government’s top law officer has argued that any ruling should wait until there is actual legislation to consider.

Many SNP activists are growing restless at the lack of progress towards independence, especially as a string of recent polls suggested more voters in Scotland now back independence than want to stay in the UK.

Nicola Sturgeon, Scotland’s first minister and SNP leader, has resisted calls for a “Plan B” to the 2014 referendum model approved by Westminster. Her new approach is in large part intended to ease party impatience while remaining committed to ensuring any vote is clearly within UK law.

Ministers in the Johnson government say they would certainly challenge the legality of any Scottish parliament bill preparing for a referendum and are confident they would win. “It would make the SNP look utterly foolish and put them further away from achieving their goal,” one UK minister said.

But SNP leaders believe a high-profile court battle, however it concludes, would further undermine support for the union.

Unionist campaigners would boycott an unapproved referendum even if it was judged to be legal. Their aim would be to drive down turnout, below the 2014 plebiscite, and then claim that a vote in favour of independence lacked legitimacy.

Douglas Ross, leader of the Scottish Conservatives, said on Monday his party would not campaign in a referendum that was not authorised by Westminster. One pro-unionist MP said: “If you wanna go ahead with it, fine go on and make a fool of yourselves. Go and get your 99 per cent support for independence with no turnout.”

Another option is for Westminster to legislate to overrule the Holyrood law. An advisory referendum organised by the Scottish government would anyway not have any binding effect and it is clear that Holyrood does not have the power to declare independence under UK law.

Some pro-union politicians hope that the prospect of an unauthorised referendum, which they describe as a “wildcat” vote, would scare off moderate Scottish voters.

But opinion polls suggest that many Scottish voters who do not currently support independence nevertheless believe their constitutional future should be decided in Scotland.

Mark Diffley, a consultant on Scottish political opinion, says indefinite rejection of a referendum by Westminster would probably push such voters toward independence. “It’s a very risky strategy.” he added.

Mr Johnson’s primary strategy is to wait. “Time is our best weapon,” one senior government official involved in the process said. “Once we pass the worst of coronavirus and [Ms] Sturgeon is off the TV every night, support for independence should start to abate. We’re not going to rush into anything.”

Some union supporters also hope the SNP’s increasingly open internal divisions will undermine its appeal. Ms Sturgeon is locked in an extraordinary feud with her predecessor as first minister, Alex Salmond, who has accused her of misleading parliament, a charge she denies.

Even without an SNP implosion, some Conservative politicians believe saying no to another referendum will be sustainable for some time — at least until there is consistently 60 per cent support for independence.

But there is no consensus about how the UK should respond more broadly to the challenge of the first sustained lead for independence in Scotland. Some union supporters want to explore devolving more powers to the Scottish parliament, including a so-called “devo max” option that would leave Edinburgh broadly autonomous.

John Lamont, the Conservative MP for Berwickshire, Roxburgh and Selkirk, said it was not clear what extra powers should go to Edinburgh. “The SNP want a drip-drip from Westminster until they ultimately get their goal of breaking up the UK,” he said.

The other option is bigger picture reform that delves into questions of British identity and the structures of the UK state. Gordon Brown, the former Labour prime minister, made this case in a newspaper article advocating abolishing the House of Lords. Mr Johnson pledged a constitutional review in his 2019 election manifesto, but this has yet to start.