A former SNP special adviser has warned Nicola Sturgeon that the prospect of a second independence referendum before 2021 appears "very remote indeed".
Campbell Gunn, who served under both Alex Salmond and Nicola Sturgeon, said a future plebiscite would be more likely to take place if the Nationalists made it a key plank of their manifesto ahead of the next Holyrood elections in just under three years' time.
The First Minister has said she wanted to see an IndyRef2 "in the lifetime of this Parliament" if the UK, as expected, leaves the European Union.
Last month Mike Russell, the SNP’s constitutional affairs spokesman, told MSPs that a potential no deal Brexit would mean “an even greater urgency to give Scotland a choice of a different future”.
Scottish ministers hope the bill laying the framework for future referendums would be passed by Holyrood by the end of the year.
Labour, the Lib Dems, and the Conservatives are all opposed to any IndyRef2, but the votes of the Scottish Greens means the SNP do have a potential pro-independence majority.
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But Gunn warned that as the power to hold legally-binding votes on the constitution is reserved to Westminster, the Scottish Government still faced significant hurdles if was to stage a meaningful referendum.
"I do not disagree with any of the first minister’s analysis of how Scotland has been treated during the Brexit negotiations," he wrote in the Press & Journal.
"However, I have to say that the likelihood of a vote in 2020 or even within the lifetime of the current Scottish Parliament session – much as I would like to see either of them – is very remote indeed. This is because such electoral matters are not in the gift of the Scottish Parliament – they’re reserved to Westminster.
"We’re currently in the midst of a Conservative leadership election with the winner automatically becoming prime minister. One by one, the candidates have stated categorically, some more vehemently than others, that they would oppose granting permission to the Scottish Parliament to hold another independence referendum. And that, frankly, is that.
"Of course, the Scottish Government could go ahead with its plans. With the support of the Greens, the SNP could win a vote at Holyrood to hold a referendum. But it would not have any legislative competence and would only be considered “indicative”, with no weight in international law. And, naturally, the Unionist opposition parties would encourage their own supporters to boycott the vote, resulting in any majority for independence being meaningless."
Pamela Nash, chief executive of Scotland in Union, said: “Campbell is right, which begs the question why is Nicola Sturgeon devoting taxpayer-funded Government resources to it and spending precious Parliamentary time on it?
“Only a fifth of people in Scotland want another divisive independence referendum within her timescale.
“She should drop her unwanted plan and get back to the day job of fixing our schools, hospitals and the economy.
“The majority of people in Scotland have had enough of her game-playing and want to remain in the UK.”