Sir John Curtice told The Scotsman that recent polls on the issue indicated that Scots continue to be divided at around 45 per cent in favour of separation and 55 per cent wanting to remain in the UK – a level unchanged since the result of the independence referendum in September 2014.

Sir John’s comments come at the end of a week in which First Minister Nicola Sturgeon issued a rallying cry to supporters to “get out there and make the case” for Scottish independence, and promised to set out a timetable for a second referendum in a “matter of weeks”.

Angus MacNeil, one of the party’s longest-serving MPs, used an appearance on Alex Salmond’s RT chat show to claim “the Scottish people have waited long enough”, while former SNP MP George Kerevan said the First Minster “should be getting on with” another vote.

Keith Brown, SNP depute leader, stopped short of demanding a second referendum, but claimed the “historic and generation-defining failure” of Westminster proved that only independence could protect Scotland’s interests.

But Sir John, a professor of politics at Strathclyde University, warned the deadlock in polling meant both sides should approach any second referendum with caution.

“We are still running at around Yes 45 per cent, and No 55 per cent which means a referendum would not necessarily be won terribly easily,” he said. “There certainly isn’t any evidence that support for Yes has particularly increased recently – but equally it also means that no Unionist could be sure that they would win.

“That’s the position we’ve been in since September 2014.”

Demands for indyref2 have increased among Nationalists since Prime Minister Theresa May’s government was humiliated on Tuesday night when MPs voted down her Brexit plan in record numbers.

The First Minister used a visit to Westminster on Wednesday to suggest details of the timetable for a second Scottish independence referendum would be revealed soon.

Sir John suggested yesterday it would be “unwise” to proceed with plans for another referendum when the outcome of Brexit remained far from clear.

He said: “It strikes me as extremely unlikely, even unwise, for the SNP to make any move on this until it is clear what is going to happen with Brexit.

“The crucial action is what will happen to the UK government in the wake of its attempt to try and get a deal through the House of Commons. It is widely agreed there is a non-trivial risk that UK government is going to fall apart over this subject.

“Given that there is no prospect of Nicola Sturgeon getting a Section 30 order out of the current parliament [giving permission for a referendum], and assuming she does not want to call for a referendum safe in the knowledge that she does not need to hold it, she is going to wait.

“She will wait because should the UK government fall, and should we face a general election – which opinion polls suggest could lead to another hung parliament – she might find herself in roughly the same position as the DUP currently find themselves in the current parliament, but obviously dealing with Jeremy Corbyn.

“There are no guarantees any of this will happen, but if we have another general election, all the balls go up in the air and we wait and see how they land. At the moment, they’re not in a position that is particularly helpful for the SNP.”

Mr Brown said Mrs May’s failure to achieve a viable Brexit deal had taken the UK to the brink of a historic crisis, with a no deal Brexit now the default without cross-party agreement on an alternative.

He said: “The Tories are guilty of an historic and generation-defining failure.

“But Labour, too, have been culpable. The SNP called for a vote of no-confidence in the UK government last year – but Labour let Theresa May off the hook and she has run down the clock.

“It has become increasingly clear that the only way Scotland’s interests can be protected – and our democratic decisions respected – is through independence.”

Scottish Conservative constitution spokesman Adam Tomkins said: “Since the result in 2014 the SNP has barely stopped talking about its bid to break up Britain. Despite this shamelessly relentless approach, support for separation hasn’t increased.

“That’s all the evidence Nicola Sturgeon should need to take the threat of another independence referendum off the table.”

Scottish Labour deputy leader Lesley Laird said: “It is deeply irresponsible for the SNP to use the instability and chaos caused by the Tories over Brexit to push for yet more instability and chaos with another independence referendum.”

Brexit '˜has not moved minds' on indyref question, says Sir John Curtice