For the last century the United Kingdom has regarded itself as a voluntary union of four home nations. Consent, rather than the force of law, has been the glue that has held us together. This is not normal. Most countries hold themselves together with something rather more robust.

In Spain, the courts, applying the constitution, ruled that it was unlawful for Catalan separatists even to hold a vote on Catalan independence. In the United States the position would be even stricter. Its leading case on the law of secession was admittedly decided in the immediate aftermath of the US Civil War, but the US Supreme Court’s authoritative decision on the matter was unequivocal. When a state becomes one of the United States it enters into ‘an indissoluble relation’ that is ‘final’, the Court ruled. There is ‘no place for reconsideration, or revocation, except through revolution’ or with the consent of the United States.

Quebec went through two secession referendums, in 1980 and 1995, after the second of which Canada referred to its Supreme Court the question of whether a province of Canada has a constitutional right to unilateral secession. The Court’s unanimous verdict was to reject as ‘absolutist’ the proposition that, even after a Yes vote indicating a clear desire to leave, there would be any legal obligation on the other provinces or the federal government to accede to the secession of a province: ‘Quebec could not purport to invoke a right of self-determination such as to dictate the terms of a proposed secession to the other parties: that would not be a negotiation at all’, the Court said. Were secession to happen, it would have to be done by negotiation and, said the Court, ‘no negotiations could be effective if their ultimate outcome, secession, is cast as an absolute legal entitlement’. And this even after a Yes vote.

We have no such legal clarity in the United Kingdom. Here, it seems, we can be threatened with referendums that have the potential to break up our country whenever a Nationalist government in a devolved administration says so.

If, as the polls predict, Nicola Sturgeon’s SNP win the Scottish parliamentary election in May and demand a second independence referendum, the Prime Minister will of course say No. He will be right to do so — now, as we battle our way out of a pandemic, is not the time for the division and rancour of another plebiscite on secession. But, in Scotland, he will be castigated for doing so. An English Tory denying Scotland her voice, there is a real risk that Boris Johnson just saying No to Nicola will push support for independence up and place the Union in ever deeper trouble. With consent fraying at the edges, what to do?

As the examples of Spain, the US and Canada show, there is nothing to require the UK to continue to think that it cannot or should not turn to the law as a constitutional safeguard. No state should be expected to have to endure repeat referendums on their very existence whenever it suits a separatist movement to hold one. The 2014 Scottish independence referendum was said by the Nationalists to be a once-in-a-generation opportunity. All well and good, except no-one ever took the trouble to define what was meant by a generation.

Perhaps we should stop being so squeamish about the law and clear the matter up. Let history be our guide. There were two Scottish devolution referendums, held eighteen years apart (1979 and 1997). There were two EU referendums held in the UK, 41 years apart (1975 and 2016). Clearly, constitutional referendums can be repeated on the same issue — on this view, no one should say ‘never’ to a second Scottish independence referendum. But giving legal clarity to the ‘not now’ arguments the UK Government will have no choice but to run if the Nationalists win another Holyrood majority next month deserves serious consideration. How much better if it wasn’t just a Tory prime minister telling Scotland that it could not re-run a vote held only a few years ago, but the binding law of the land.

The United Kingdom is perfectly entitled to take to the law to defend itself. Again, lessons from constitutional law elsewhere in the world are instructive. The new South African constitution, lauded around the world as that country emerged from the pit of apartheid, provides for three spheres of government — national, provincial and local. Section 41 of the constitution provides that all spheres of government and all public bodies ‘must preserve the peace, national unity and the indivisibility’ of the country and ‘must be loyal to the constitution’.

The United Kingdom needs a new Act of Union to set out, authoritatively, the rare circumstances in which one part of the country can seek lawfully to secede. As well as defining how frequently referendums on such a matter may be held, the law could at the same time impose obligations on governments and public bodies throughout the land to act with fidelity towards — and not to undermine — the territorial integrity of the country. Nationalists will howl with rage at this proposal. In response to them I would just gently point out that their beloved European Union includes within its treaties a like provision: ‘Member states shall facilitate the achievement of the Union’s tasks and refrain from any measure which could jeopardise the attainment of the Union’s objectives’.

Time to copy and paste this idea from the EU’s rulebook into our own: ‘devolved administrations shall facilitate the achievement of the United Kingdom’s tasks and refrain from any measure which could jeopardise the attainment of the United Kingdom’s objectives’. What Europhile Nationalist could object to that?

Adam Tomkins is a Professor of Law at the University of Glasgow and a Senior Fellow at Policy Exchange.