A 'de facto' referendum on independence is “just about the worst possible way to settle the constitutional future of Scotland”, the SNP’s longest serving MP has said.

Pete Wishart said the route proposed by Nicola Sturgeon had never been tried in a western democracy before and carried “huge” risks.

These included Westminster refusing to engage with it and ignoring the outcome, as well as the “sheer challenge” of getting a majority of the vote at the next general election.

However he also said it was now the “only way” to settle Scotland’s constitutional future after the UK Supreme Court ruled Holyrood cannot hold a vote without Westminster consent.

Writing in the National, the Perthshire MP said: “If we fail to pull it off, we may surrender our leading position in Scotland and could possibly kill off any hope of independence being secured in a “real” generation. 

“It is a massive gamble – with the emphasis on massive. But what else can we do?”

Ms Sturgeon proposed a de facto referendum last June when she announced the Supreme Court was being asked to rule on whether it had the power to stage Indyref2 unilaterally.

She said that if the Court ruled against her, she would fight the general election due by January 2025 as a de facto referendum on the single question of independence.

After two days of hearings in October, the five justices who heard the reference ruled unanimously that Holyrood did not have the power, as a referendum about whether Scotland should be independent strayed into the Westminster-only area of the Union.

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The SNP is to hold a special conference on the de facto referendum Plan B on March 19.

There are many in the party who are sceptical about whether it can work in practice, with the SNP Westminster leader Stepehen Flynn reportedly among them.

The SNP’s ruling body, the National Executive Committee, is due to discuss the subject this Saturday.

Mr Wishart said that despite his misgivings about the de facto referendum idea, action was required in the face of the UK Government refusing to revisit the No vote of 2014. 

He said: “A move has to be made and in the face of UK intransigence, it is incumbent on us to move the pieces and push this forward.

“We now know there are no other legal means to test Scottish public opinion

“The Supreme Court concluded we do not have the legislative powers in our Parliament to hold our own referendum and it seems apparent that the UK Government will never agree to a Section 30 order [transferring powers] or the devolution of legislative powers.

“This then is the only way forward and for all its difficulties, challenges and drawbacks it is our only, and possibly best last chance.”

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He said one upside was that using a UK election would give the issue publicity.

But there were also drawbacks, including 16 and 17-year-olds and EU nationals not being able to vote in general elections.

However, using a Holyrood election instead would have an “obvious difficulty” as it would in 2026 and there was “a greater risk of antagonising an electorate which would want the opportunity to elect a Scottish Government focused on a full delivery of services”.

He dismissed outright the idea - proposed by fellow SNP MP Angus MacNeill and the Alba party - of dissolving Holyrood early to trigger a snap election.

“That would only lead to First Minister Douglas Ross,” he said. 

“The Scottish people would be likely to punish any government that simply walked away from its responsibilities of governing at a time when that is so crucially required. 

“The optics of this on the people we would need to win over would be simply awful.”

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He concluded: “The way to solve the constitutional future of a nation is to have a referendum in which both sides of the debate put their case in respective campaigns. 

“This is what the people of Scotland voted for and it is what they should secure.

“We won’t have a properly agreed referendum because the UK Government defiantly refuse to engage in even the broadest discussion about a referendum and simply ignore the will of our Parliament.

“In the face of this intransigence there are therefore only two options. One is to do this. The other is to simply give up, do nothing and continue with this constitutional deadlock.

“After coming this far and getting so close I’m not for giving up.”