Mrs May has said she plans to table Article 50, formally notifying Europe of the UK’s intention to leave, by the end of March which means that Britain could be out of the EU by March 2019.

If Ms Sturgeon waited until after that date Scotland, as well as the rest of the UK, would be out of the EU.

However, if a referendum before that date – say September or October 2018 – resulted in a vote in favour of independence senior SNP figures believe it would enable Scotland to remain in the EU whilst it negotiated its exit from the UK.

A major problem for the First Minister is, of course, the fact that the Scottish Parliament does not have the power to call a referendum – only Westminster can do that. But the Commons can devolve that power to Holyrood through what’s called a Section 30 transfer, which is what happened to allow the 2014 referendum.

Then conditions were agreed between the two parliaments about such things as the question on the ballot paper, the precise timing of the vote and the minimum age of those allowed to vote.

Similar negotiations would have to take place in the event of a demand for another vote.

Mrs May would be within her rights to refuse consent for another vote but she’d be unlikely to do so if there was a significant head of steam behind the Nat demand.

That said, she could impose conditions, for instance over the date – refusing to allow an independence vote until after March 2019 by which time the UK would probably be out of the EU and over the wording of the question on the ballot paper.

Whilst there is widespread enthusiasm amongst most SNP activists for another referendum it is not shared by many senior figures in the party.

With a former leader and deputy leader, as well as a number of former ministers, in the critics’ ranks their views must be borne in mind by the First Minister.

They say that she’s boxed herself in with her calls for a referendum and that another defeat would kill for ever the chances of Scottish independence. Most also point out that Scotland’s economy is worse, not better, now that it was in 2014.