NICOLA Sturgeon may have called a second independence referendum too soon to win it, according to Scotland’s most respected historian.

Professor Sir Tom Devine, who was a prominent Yes voter in 2014, said the First Minister may have been wrong to announce her plan before the uncertainties of Brexit were resolved.

In an interview with BBC Radio Scotland, Professor Devine also said he had “very great doubts” that Theresa May would negotiate Brexit in line with her 18-month timetable.

Indeed, he said was “at least 50-50” that Brexit would fail and the UK would stay in the EU.

He also said the Prime Minister and her Conservative predecessors were contributing to the failure of the Union by being “obdurate” and insensitive to the devolved nations.

The First Minister announced earlier this month that she wanted a second referendum between autumn 2018 and spring 2019, so that Scots could choose between Brexit in the UK and independence and a closer relationship with Europe.

Asked if Ms Sturgeon had played her hand too early given the many unknowns around Brexit, Prof Devine said: “I think so, yes. That would be my view. She had to indicate a particular position, albeit in vague terms, because she like May was becoming boxed in. If she hadn’t done something or said something her credibility would have collapsed.

"The big question for Nicola Sturgeon is - if Theresa May adheres to her position, where does Nicola Sturgeon go? What ammunition has she got?

“Does she go back to history and look at the way the Irish MPs behaved in the later 19th century, by resigning en masse from parliament? Does she call a Scottish election?

“I think the possibility of doing a mock poll is ridiculous, because Unionists will simply ignore it. So it’ll be a very big test for the First Minister.”

Asked what a second No vote would mean for Ms Sturgeon and her cause, he said: “It’s Götterdämmerung [apocalypse]. In a sense, 2014 wasn’t quite as serious as that, because that was the first time. If there’s a second vote and it’s still No, that’s it.

“Automatically, Sturgeon will resign, her career’s over. At the same time, certainly for people of my generation and the generation before me, that’s it over for ever and a day.”

Agreeing with recent comments by psephologist John Curtice, he said the SNP might have been better waiting for demographics to deliver independence, as there was overwhelming support among the under-45s who would outlive the pro-Union over-65s.

A growing resentment of Scotland among English voters, and a preference for Brexit over the Union, was also contributing to the long-term dissolution of the UK, he said.

Prof Devine, the Sir William Fraser Professor of Scottish History and Palaeography Emeritus at Edinburgh University, was scathing about the Prime Minister’s position.

He said Mrs May's suggestion that Brexit might strengthen the Union was “ludicrous”.

He said: “As we speak, I think that’s a ludicrous position to take up, it’s almost comic. Because it’s precisely the fear of Brexit that’s resulted in another crisis in the Union.”

He was sceptical that Mrs May would deliver on Brexit, as there was “a whole forest of complexities” to negotiate over an 18-month timetable.

“Personally, I don’t think that’s possible,” he said, agreeing with the former UK ambassador to the EU, Sir Ivan Rogers, who predicted it would take at least 10 years.

“I’m doubtful whether there will be a Brexit at all. There are simply so many obstacles, so many of them not yet revealed. Every member state has a veto.”

He said if MPs were denied a vote on the final deal, it could result in another legal challenge.

“I think it’s at least 50-50 that there will not be a Brexit at the end of the process, which has huge implications for the Scottish situation and the timing of any future referendum.”