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A level of 60% support for Scottish independence over the period of a year has been identified as a benchmark in making the decision over a second referendum, senior SNP sources say.

The figure is a "trigger point", but will not be publicly acknowledged, sources told the BBC's John Pienaar.

Prof John Curtice said it was the level the party should be thinking about.

An SNP spokesman said there would only be a second referendum if there was clear evidence of a shift of opinion.

SNP leader Nicola Sturgeon will not be drawn on whether a second referendum will take place while she is in charge.

But sources close to Ms Sturgeon said the benchmark was vital to ensure support for independence had become the "settled will" of the Scottish people.

"Six months of polls won't be enough," said a senior SNP figure, involved in the discussions.

Barrage of attack

Speaking on 5 Live's Pienaar's Politics, Prof Curtice, who is president of the British Polling Council, said: "It's entirely the kind of benchmark they should be thinking about.

"I think some of us said, not long after last year's referendum, that this was the kind of scenario the SNP needed to see in play before they could seriously contemplate a referendum."

He added: "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'.

Image caption SNP leader Nicola Sturgeon said a second independence referendum would only come when the time was right

"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.

"Why 60% - 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."

An SNP spokesman said: "As the FM set out there will only be a second referendum on independence if there is clear evidence of a shift of opinion.

"The far more urgent question is whether the UK government will honour the vow by strengthening the Scotland Act, end the unnecessary ideological austerity drive that is hurting low income households and act to protect Scotland's place in the EU."

'Clear evidence'

On Thursday, Ms Sturgeon told the BBC that even a "thumping win" at next year's Scottish elections would not be enough to push for a second referendum.

Earlier, she had opened her party's Aberdeen conference calling on those against independence to vote SNP.

In an interview with the BBC's political editor Laura Kuenssberg, Ms Sturgeon said the timing of a referendum vote would be "down to whether we judge, I judge, that people who voted 'No' last year have changed their minds".

And during her speech at the conference on Saturday, she reiterated that a second independence referendum would only come when the time was right.

She said the time for another Scottish independence referendum would be "when there is clear evidence" that opinion had changed and the majority of people in Scotland wanted it.

She added: "Independence matters and we will never waver in our commitment to it. But what we say about jobs, schools and hospitals matters just as much to people across Scotland."