SCOTLAND’S right to self-determination is a “political reality” and Boris Johnson will not be able to say no to a referendum for long, according to a leading constitutional expert.

Dr Alan Renwick, deputy director at University College London’s Constitution Unit, also said the UK Government should care about the “future wellbeing of people across these islands” by not taking a “narrow self-interested perspective”.

His comments were made during an online discussion held by the Constitution Unit last week called Can Johnson prevent Indyref2?

Renwick said he did not have a “general theory” on who should have a right to self-determination.

But he added: “I think it is just a matter of political reality that in the UK across the political spectrum, Scotland’s right to self-determination has been recognised.

“It has been recognised by Conservative prime ministers, by Labour prime ministers, by Unionist leaders in Scotland.

“I don’t think you can get away from it, I think it is simply a reality that has to be accepted.”

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Renwick said while the Prime Minister could say no for a while, that could not go on for very long – and he was likely to then have to agree to a referendum which he would place conditions on.

He said: “I think at some point unless opinion changes quite dramatically he does start having to say yes – ‘but’.

“At that point he does have to be very careful about what the ‘but’ is.

“On the one hand it might be tempting for the UK Government to play really hard and to put all sorts of conditions in here that would be very difficult for the Scottish Government.

“On the other hand if it looks like the UK Government is being really self-interested in this and is not having adequate regards for Scotland’s interests, then that risks antagonising people even further.”

He said the best strategy would be to try to engage public opinion to find out what options the country wants.

He argued against “sleepwalking” into a binary choice between the Union and leaving the UK, saying another option could gather wider support.

But he added: “Of course potentially the answer that comes out of that process is yes we want a binary referendum with independence as one of the options.

“So it is a very risky course for a government to take – whenever you bring the public in in that way you need to accept they might not give you the answer they want.”

Renwick, who also leads the working group on referendums in Ireland, also said the approach taken to Northern Ireland was relevant to the Scottish situation.

“My view is the UK Government ought to care for the future wellbeing of people across these islands and that is best served by not taking a very narrow, self-interested perspective on what is the right approach here,” he said.

“The situation in Northern Ireland is different, but relevant here. Since the early 1990s the UK Government has taken the view it does not have a selfish interest in the question of whether Northern Ireland remains in the UK or not.

“If you accept the principle of self-determination for Scotland, then of course you can still have a view on whether you want Scotland to be in the Union or not, just as you can have a view on whether you like Northern Ireland being in the UK or not.

“But as the Government you also have to be willing to stand back and allow that choice to be made in a fair way that people will regard as having been fair after the event.”

Aileen McHarg, professor of public law at Durham University, said unless there was willingness to give up the sovereignty of the UK Parliament, any constitutional “third way” was a non-starter.

She said scepticism about the 2014 Vow, for example, was likely to mean a promise of more devolution on its own would not be enough.

And putting forward a third option will be more difficult in light of recent issues such as the Sewel convention being ignored, which states UK Parliament will “not normally” legislate on devolved matters without the consent of the devolved legislatures.

“I don’t think very many people will be persuaded by those symbolic guarantees any more, you would have to have real enforceable constitutional guarantees which would require a fundamental recasting of the UK constitution,” she added.