Nicola Sturgeon, Scotland’s first minister, vowed she would seek to hold a “legal” second independence referendum, if necessary without UK government approval, if she wins elections for the parliament in Edinburgh in May.

Ms Sturgeon’s comments came as her governing Scottish National party held a virtual national assembly for members to discuss “alternative routes” to a referendum should Boris Johnson, prime minister, refuse to approve a vote.

In 2014 Scottish voters backed staying in the UK by 55 to 45 per cent, but dissatisfaction with Brexit — which 62 per cent opposed — and perceptions that Ms Sturgeon has handled the coronavirus crisis better than Mr Johnson have since helped build support for independence.

A Panelbase poll published on Sunday for the Sunday Times found that 52 per cent of Scottish voters willing to express an opinion would back ending the three century-old union with England. It was the latest in a series of surveys over the past year that suggest a second referendum would result in a majority for independence.

Mr Johnson has suggested that Westminster would not allow another independence referendum until the 2050s, but the SNP announced on Saturday that even without UK approval it would seek to pass a bill to organise such a vote if there was another pro-independence majority in the Scottish parliament after elections scheduled for May.

“I want to have a legal referendum, that’s what I am going to seek the authority of the Scottish people for in May,” Ms Sturgeon told the BBC on Sunday. “If they give me that authority that’s what I intend to do: have a legal referendum to give people in Scotland the right to choose.”

The Scottish government and some legal experts have in the past argued that legally Edinburgh can unilaterally hold an advisory independence referendum under UK current devolution law.

Under a plan announced on Saturday, the party will seek to pass a bill in the Scottish parliament setting the date and the question for a referendum.

Michael Russell, SNP president and Scotland’s constitution secretary, said Westminster would then have to decide whether to give approval 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”.

The new SNP plan marks a concession by the party leadership to pressure from members for a referendum “Plan B”, but many are likely to say it does not go far enough.

The UK government insists that an unapproved independence referendum would be illegal under devolution law and many expect London would win any legal challenge. If necessary, Westminster could also change UK law to make clear that Edinburgh cannot proceed.

Angus MacNeil, an SNP member of the UK parliament who has been critical of Ms Sturgeon’s approach and believes the May election itself should be treated as an independence plebiscite, dismissed the new plan as based on “hope”.

“Independence supporters must grasp that on balance any referendum is extremely unlikely to happen in the next five years, Westminster or the courts will see to that,” Mr MacNeil wrote on a pro-independence discussion website.

But while Mr MacNeil said the National Assembly had marked a “clear shift in thinking” away from reliance on UK approval, Pete Wishart, another SNP MP, tweeted that the online event had shown up the “naive assertions about international ‘recognition’ and democratic legitimacy” of those who wanted to treat May’s election as a plebiscite.