First Minister Nicola Sturgeon has been playing down the prospect of an imminent re-run of last year’s ballot - which saw a 55%-45% victory for the unionists.
She has suggested that a new vote could be triggered if the coming in/out referendum sees Britain exit the EU in defiance of Scottish wishes.
But senior SNP sources are said to have told BBC Radio 5 Live’s Pienaar’s Politics that they have also set a “test” of polls showing at least 60% support for independence for more than a year.
British Polling Council president Professor John Curtice said he agreed that such a level of sustained support would be required to ensure Scotland voted to leave.
“I think one of the things that’s forgotten about the referendum last year is that there had never previously been a period in which the opinion polls had consistently pointed to a majority in favour of yes,” he told the programme.
“There really isn’t much point in the SNP holding a referendum until it’s clear that there is a majority - a sustained majority - in favour of doing so, because otherwise the serious risk is loss.
“There clearly is a serious prospect that that figure will come down under the sustained barrage of attack that there will undoubtedly be on the independence project in the event of a second referendum.”
Former SNP leader Alex Salmond said he was “not privy” to Ms Sturgeon’s requirements for seeking a fresh referendum.
He insisted the party was “content to leave the timing of the referendum to Nicola Sturgeon and of course to the Scottish people”.
However, Mr Salmond, now MP for Gordon, argued that the UK leaving the EU looked “more of a possibility now than it did six months ago”.
“The Scottish polls on Europe show a very convincing majority for staying in, the UK - or in this case, the English - polls on Europe show a distinct narrowing,” he said.
When asked whether he thought the Westminster Government would block another referendum in Scotland - as David Cameron has suggested - Mr Salmond dismissed the threat as “hot air”.
“It would be impossible politically ... I think the pass has been sold for, how shall we put it, a prime ministerial veto on Scottish ambitions,” he said.
“He’s not going to be around. David Cameron is very much in his final stages as Prime Minister.
“On this matter as in many others, I think increasingly people will stop listening to David Cameron and start looking for somebody who might be in power at that time.”
Mr Salmond went on: “Trying to avoid it against a mandate from the Scottish people in a Scottish election and a majority in the Scottish parliament would be unreasonably trying to avoid it.”