Over the weekend, the SNP set out an 11-point routemap to an independence vote. It states another vote could be held if a pro-Yes majority is returned to Holyrood in May, regardless of whether a Section 30 order is granted by Westminster. The document explains it would then be for the UK Government to decide if it wanted to try and stop it through the courts.

Professor Marc Weller of Cambridge University, formerly a senior legal adviser to the UN, says Holyrood can point to the material change in circumstances caused by Brexit, as well as appealing to the “principle of self-determination in international law” if it has to make its case for another referendum in court.

Writing for the Scotsman, he explains that claims referendums are only appropriate “once in a generation” – made repeatedly by Boris Johnson and his ministers – are not borne out in practice, pointing out New Caledonia repeated its plebiscite on independence from France just two years after the initial poll of 2018.

The international law expert also warned Unionists that boycotting a second referendum would not necessarily invalidate the vote – a strategy suggested by Scottish Tory leader Douglas Ross if Westminster does not give its consent. Nor will they be able to deny a ballot “indefinitely”.

The SNP say the analysis shows "just how weak the Tories’ case is in trying to block a referendum".

READ MORE: Douglas Ross calls for indyref2 boycott if it's held without UK consent

Weller's article begins by debunking claims about a "once-in-a-generation" plebiscite.

"Holding a second referendum after some seven or, in actual fact perhaps more likely eight years, seems quite reasonable in a democratic society," he writes. “This would be two full electoral cycles for most states. In addition, Scotland can point to the fundamental change in circumstances brought about by Brexit.”

He believes if opposed by Westminster, the case for indyref2 is likely to be addressed by the Scottish Court of Session and then by the Supreme Court. Setting out Scotland’s possible legal avenues, the barrister states: “Scotland can also appeal to the principle of self-determination in international law.”

The professor explains this principle was recently affirmed by the International Court of Justice in advisory proceedings brought by the United Nations General Assembly in a case involving the Chagos Islands in relation to the UK.

Although it is disputed whether this principle applies in non-colonial cases, Weller writes: “In this instance it cannot be doubted that Scotland is inhabited by an established nation historically tied to a clearly defined territory. Through devolution this distinct and territorially defined population has demonstrated the ability to govern itself.”

Scotland’s right to independence is "established informally in UK constitutional practice", he notes.

"International law recognises such a constitutional grant of authority, whether made express in the constitution or implied in constitutional practice," Weller writes. "This internal, constitutional entitlement is then reflected in the international law doctrine of constitutional self-determination.”

READ MORE: Tories 'will be taking millions of Scots to court' if they challenge indyref2

He adds: "According to the legal doctrine of uti possidetis, the right to self-determination through a referendum outside of the colonial context applies only to larger, constitutionally recognised units – in this instance Scotland."

When this principle is valid, it is "generally" applied through a referendum.

"Hence, if it is clear that Scotland is entitled to opt for independence, and if the means to do so is through an act of will of its population, then it follows that the central government should not be able to obstruct the implementation of that right by refusing a referendum," Weller explains.

Douglas Ross has called for a boycott of indyref2 if it doesn't receive Westminster's blessing

The professor sets out four conditions that must be met for any future referendum, based on conclusions reached by the Canadian Supreme Court in relation to Quebec.

Firstly, the referendum question must be “clear and unambiguous”.

Secondly, the referendum process must be open and inviting to all sides. "The EU Badinter Commission found in relation to the former Yugoslavia that opponents of independence must be allowed to campaign and to participate on equal terms with proponents," Weller states.

“Where this second condition is fulfilled, the unionists cannot obstruct the referendum through a boycott. The vote will count, even if they chose not to participate, unless total voter participation falls below a reasonable percentage.”

Thirdly, the result must be clear – usually 50% plus one.

“And fourth, even if the pro-independence side wins, it cannot simply walk away and declare independence,” the professor states. “It, too, must take account of the interests of the other side and negotiate in good faith about a balanced and reasonable divorce settlement.”

The professor concludes: “It will not be possible to deny a referendum indefinitely. On the other hand, Scotland can only succeed as a state within the EU if it can demonstrate that it has obtained the fullest consent from Westminster for its potential journey towards independence. This gives both sides equal weight in the discussions that will follow upon the elections in May.”

The analysis was welcomed by the SNP.

The party's depute leader, Keith Brown, told The National: 

SNP depute leader Keith Brown said: "This expert opinion shows just how weak the Tories’ case is in trying to block a referendum, and how the UK Government risks falling on the wrong side of the principles of democracy it purports to champion elsewhere in the world.

“Our ‘Roadmap to Independence’ clearly sets out that an independence referendum 'should be held after the pandemic, at a time to be decided by the democratically elected Scottish Parliament.'

"At May's election, we will make the case for Scotland to become an independent country and seek a clear endorsement of Scotland’s right to choose our own future.

"And, if the people of Scotland vote for a referendum at the Holyrood election next year, then there will be a referendum."

He added: "No matter how much Boris Johnson copies Donald Trump by trying to deny free and fair election results, the people of Scotland will decide their own future."