THE lead official negotiator for Prime Minister David Cameron ahead of the 2014 independence referendum has said he is "certain" Boris Johnson's Government has "blocked off" all legal routes to Indyref2 next year.

Professor Ciaran Martin's intervention will come as a blow to Nicola Sturgeon who is due this Tuesday to set out to Holyrood her route map for a new vote in October 2023.

She published the opening "scene setter" in a series of papers to update the case for independence earlier this month but faced questions over how she would deliver a legally binding vote given the UK Government has repeatedly refused to agree to a new referendum.

Mr Martin, Professor of Practice in the Management of Public Organisations, at the Blavatnik School of Government at Oxford University, said while "some form of consultative referendum might be possible legally" - though he was sceptical - he was "certain that for now any and all lawful, democratic routes to independence are blocked off by the UK Government".

HeraldScotland:

Professor Ciaran Martin

However, he questioned the wisdom of the UK Government's "blanket refusal" to contemplate a new referendum, warning the Pro-UK side of the debate that it “carries real strategic” risk over the longer term and that such a position was "not eternally sustainable".

He referred to the events surrounding the devolution referendums and pointed to the outcome of the 1997 vote when 74 per cent of Scots backed devolution including 63 per cent who said the parliament should have tax raising powers.

The vote came 18 years after 52 per cent of Scots voted in favour of an assembly, however a legal required had been put in place that the vote had to be approved by 40 per cent of the total registered electorate. It meant that the 52 per cent majority wasn't sufficient for devolution to go ahead then as, taking into account a turnout of 64 per cent, it represented only 33 per cent of the total registered electorate.

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“The goal of the Scottish Government and the wider independence movement is not to hold a referendum. It’s to achieve independence. Whilst some form of consultative referendum might be possible legally, though I’m sceptical, I am certain that for now any and all lawful, democratic routes to independence are blocked off by the UK Government," he told The Herald on Sunday.

"The UK Government is under no obligation to set out under what circumstances Scotland might become independent, and it is refusing to develop a policy or enter into any discussions about what such a framework might look like. So at the moment, while in principle Scotland can become independent, in practice it can’t, no matter how it votes at elections, or how often it does so."

He added: "The UK Government is perfectly entitled to hold this position, even though there is at least some evidence of a mandate for a second referendum in this Scottish Parliamentary term – the election of a Holyrood manifesto on that platform, which was enough in 2011. Mr Martin went on to say that with polls showing only around 29 per cent of voters wanted a referendum next year the UK Government "appears to be gambling on" is there being no great demand for another referendum in Scotland right now,

"What the UK Government appears to be gambling on is that despite these [election] results there is no great demand for another referendum in Scotland right now, so they can face down this Holyrood majority without any undue difficulty, and without causing an increase in support for independence," he said.

"They are also gambling on thinking that not many Scottish voters particularly care if they don’t say anything about under what conditions in future a referendum might be held. According to the polls, they are probably right: most Scots don’t seem to want a referendum in 2023. And UK ministers look set to enjoy the short-term discomfort of Scottish ministers being unable to deliver what they promised at the election."

He continued: "But that’s in the short term. In the longer term, and I doubt that many in Whitehall are thinking about the longer term, a blanket refusal even to contemplate or discuss the issue about how a further independence process and referendum could work carries real strategic risks.

"Put it this way: those who hated the idea of devolution thought they were being awfully clever in sabotaging the 1979 referendum with a turnout threshold. And in the short term they were. Then came a long period of Conservative rule where Scotland felt ignored and every path to devolution was blocked.

"Put those two things together and less than two decades later, as the 1997 referendum showed, devolution went from being something Scotland was nervous about and split over to something that was the settled will of the Scottish people. And part of that narrative was the undemocratic nature of what had happened in 1979 and subsequently."

He added: "I wonder if history will look back on this period as one where the UK Government was trying to be too tactical and not strategic: by using its legal power to wish away the prospect of an independence vote in the short term it strengthened the chances of a vote in the long run, with less chance of success than it has now.

"In any case at some point something has to give, whether that’s soon, or at a distant point in the future. That’s a matter of simple logic. The current constitutional position in the UK is: first; Scotland is in principle allowed to choose independence, but second; no matter how Scotland votes, or how often, there is no practical, lawful and democratic path to achieve independence.

"At some point one of those two things has to change because such a self-evidently contradictory position is not eternally sustainable.”

A Scottish Government spokesman said: “People in Scotland have voted for a Parliament with a clear majority in favour of independence and with a mandate for an independence referendum.

“The Scottish Government is committed to a referendum that is constitutional and lawful and which will be recognised both here and abroad.”

Responsibility for constitutional matters was reserved to Westminster when Holyrood was set up in 1999.

This means a court is likely to strike out any attempt to pose the question put in the 2014 vote, when Scots were asked “Should Scotland be an independent country?” and 55 per cent answered “No”.

However, reports have suggested a so-called advisory referendum, testing support for independence or asking voters if they believe the Scottish Government should begin independence negotiations with London, could be allowed.

Mr Martin told The Sunday Times last week: “The talk in Edinburgh circles is of a clever legal wheeze where softer legislation is drafted.”

Responding to Mr Martin's suggestion Matt Qvortrup, professor of political science at Coventry University, said that although he believed the Scottish Government had the mandate to hold another constitutional ballot, it did not have the powers to force it through in the face of objections from Westminster.

“Any decision of that nature would be met with a judicial review challenge,” he told The Times. “It is very likely it would be shut down by any court. From a legal point of view it is very difficult to see how it would work, because it is not within the rule of law.”

Constitution Secretary Angus Robertson said last week the Scottish Government will continue to press for a repeat of the “gold standard” set by the 2014 independence referendum process.

A limited selection of legal advice was published earlier this month after a lengthy freedom of information battle with the Scotsman newspaper, but the key question of whether the Scottish Government has been advised that putting forward a bill for a second independence referendum is within the powers of the Scottish Parliament was not included in the disclosure.