THE SNP cannot back down from using the next general election as a 'de facto independence referendum' without damaging Nicola Sturgeon's position as leader, according to Scotland's top scholar on the SNP.

Professor James Mitchell, chair of public policy at Edinburgh University, said it would even be difficult for the party to amend the policy without weakening her role, especially so soon after a rebellion last year among her party's MSPs over the gender recognition reform bill.

His warning came as the SNP's ruling body, the National Executive Committee (NEC) met with the First Minister yesterday to agree the text of the resolution to be debated at the party's special conference in March.

The motion to be put to party members was agreed, with Ms Sturgeon's stated preference - if no section 30 order is granted for an agreed referendum - to use the next Westminster election, expected 2024, as the de facto vote.

But in a change of plan, members will also be given a second option of using the next Holyrood election, due in 2026, for the same purpose.

A further option set out in the motion to be debated by members states that the "SNP will contest the next UK General Election, on whichever date it is called, on the issue of securing agreement for a transfer of power to enable the Scottish Parliament to legislate for a referendum.

"The SNP will make clear that it is asking people to vote SNP in that election to indicate support for a referendum."

Professor Mitchell told The Herald on Sunday: “The SNP will find it difficult digging itself out of the ‘de facto referendum’ hole – to reverse her June announcement or even find some face-saving position would be seen as a blow to [Nicola Sturgeon’s] leadership and coming soon after some of her MSPs rebelled on gender recognition would be potentially very damaging."

Ms Sturgeon announced the plan to use the next Westminster election as a de facto referendum in June as she set out a route ahead for a new vote on independence and again put the policy forward after the Supreme Court's ruling in November that Holyrood cannot hold a referendum without the UK Government's agreement.

Under her proposal if more than half the electorate vote for pro-independence parties in the next general election she would seek to begin independence negotiations with whoever is Prime Minister at the time.

However, there is much internal dissent over Ms Sturgeon’s proposal, even from party loyalists.

Last week Pete Wishart, the SNP's longest serving MP, described a de facto referendum as a 'massive gamble' that risks killing off independence for a generation and the SNP’s position as Scotland’s main party.

Mr Wishart said he backed Ms Sturgeon’s plan to try to turn the next general election into a single issue on the integrity of the UK despite it being “just about the worst possible way to settle the constitutional future of Scotland”.

He conceded that the UK Government could ignore the First Minister even if she achieved her threshold of more than 50 per cent of the popular vote being cast for pro-independence parties.

Stewart McDonald, the Glasgow South MP who is seen as Sturgeon loyalist, said last weekend that it would be a “mistake” to focus only on settling the independence question as quickly as possible.

Jim Sillars, a former SNP deputy leader, a prominent critic of Ms Sturgeon, yesterday withdrew his initial support for the plan, telling The Herald it was a "hostage to fortune" and should be dropped.

He instead said the party should focus on building support for independence to reach around 60 per cent and then seek an agreed referendum with the UK Government.

His view echoed that of former SNP top strategist Stephen Noon who last November, the day before the Supreme Court ruling, called for a pause in the demand for a referendum even if the judgement went in the Scottish Government's favour.

Professor Mitchell, who is the author of many books on Scottish politics including on the Hamilton by election in 1967 won by the SNP's Winnie Ewing, said the debate had echoes of discussions the SNP went through in previous decades between its 'fundamentalists' and 'gradualist' wings.

He put Ms Sturgeon is the contemporary "fundamentalist" camp because of her focus on a new referendum and apparent absence of other constitutional reform plans which could see independence achieved through a more gradual approach.

The leading academic coined the terms in the 1980s to describe two factions in the party. Fundamentalists were figures in the party who believed in "independence nothing less" and who were opposed to devolution seeing it as a distraction, if not a trap to the independence campaign.

Fundamentalists saw other issues - whether it was the economy, health services, education for example as well as devolution - as secondary or subservient to the cause of independence. Gradualists on the other hand saw devolution as a stepping stone to independence.

Professor Mitchell said following the defeat of the 1979 devolution referendum and subsequent loss of nine of the SNP’s nine MPs in 1979 the main tension in in the party focused on devolution and how it should respond to Labour’s proposals for devolution during the 18 years of Conservative governments after 1979.

"The SNP eventually, rather late in the day and after much hesitation, decided to support devolution in the 1997 referendum," he said.

"Many thought that the fundamentalist versus pragmatist tension was over when devolution was established - but variations of the tension continue to exist and have since arisen in a new form."

"But the same key tensions still exist and the same attitudes prevail in sections (especially the leadership) of the SNP: independence nothing less and independence nothing else.

"What does it do in the event of a reforming Labour Government? Does the SNP support Labour plans to replace the Lords with a Chamber of Nations and Regions for example? Or is it independence nothing less? That question will have to be faced if Labour is returned to power and likely before if the public believes a Labour govt looks likely. Or will it simply demand another independence referendum?"

He continued: "The intriguing feature and perhaps most significant aspect of this new fundamentalism is the emphasis is on demanding a referendum rather than making the case for independence.

"The SNP has to some extent turned into a Referendum Party rather than an independence party. So the new fundamentalism is finding expression more in the demand for a referendum avoiding difficult questions that will need to be answer - on currency, relations with rUK, the economy, pensions etc - but also as part of the blame game the SNP Government indulges in, blaming the SNP’s failures in office on London. The referendum has become the fundamentalists way of avoiding difficult questions on independence and challenges in its record in office."

He added: "The SNP membership has traditionally been fairly pragmatic though activists tend to veer off into a fundamentalist position under certain conditions - notably after a heavy defeat as in 1979 or under the influence of a hardline leader.

"Three key factors have influenced the new fundamentalism: a misreading of the public mood following the Brexit referendum ie the assumption that this would lead to a considerable and consistent lead of around 60 per cent support for independence; a leadership that prefers campaigning to governing; and fear and uncertainty on how to address key questions about independence. The referendum has become the SNP’s preferred position as it avoids difficult questions, offers what some see as a quick fix when in truth there is no quick fix."

Asked about who is was referring to in the leadership, he said: "The key figure is Nicola Sturgeon... One change that has emerged post- John Swinney’s leadership is that the SNP has become much more top down, started under Alex Salmond but taken much further under Sturgeon.

"The SNP has been transformed post-devolution from being a highly decentralised participatory party into one that is remarkably deferential to the leader though there are signs that deference is waning at least on the fringe as more activists privately acknowledge that its record in office is unimpressive and questions are being asked about the ‘de facto referendum’."

An SNP spokesman said: "There is no escape from the chaos of Westminster under either the Tories or the pro-Brexit Labour party. That is why the only way Scotland can begin to flourish is by becoming an independent country."

"Members of the SNP will have the opportunity to debate and bring forward amendments to the plans to hold a de facto referendum at the party's special conference in March."