In June 1996, the government and the judiciary were locked in a state of what Lord Irvine, then Labour’s shadow lord chancellor, described as “unprecedented antagonism”. Michael Howard’s plans for tougher sentencing would place “legislative straitjackets” on the judiciary, said Irvine, and as the government claimed more powers over judicial decisions, judges could be provoked to “to reverse government policies” in “a judicial invasion of the legislature's turf”. The separation of powers, Irvine said, was a “delicate balance” that relied on each branch knowing its constitutional limits. The speech is remembered today for its message to judges: stay out of politics.

Today, as 11 Justices of the UK Supreme Court prepare to decide the most important constitutional case in a generation, a similar turf war has erupted. The problem is, nobody knows exactly whose tanks are on which side of whose lawn. This dispute over constitutional geography is at the crux of the fierce battle over Boris Johnson’s prorogation of parliament, and the Supreme Court will have to decide where the borders of our unwritten constitution lie.

The Scottish Court of Session and the Divisional Court of England and Wales have already provided two different maps of the constitutional territory, by offering two different judgments on whether the government’s advice to the Queen to prorogue parliament for five weeks was lawful. It is now up to the Supreme Court to decide whether the Prime Minister has, in using prerogative powers to avoid parliamentary scrutiny, breached our unwritten constitution.

As the justices will be acutely aware, in taking on their proper constitutional role they will have to leave behind the cosy anonymity they have enjoyed for decades. This will have consequences which will strain our separation of powers system, perhaps to breaking point. Nonetheless, is now time for the Supreme Court to come of age.

The key decision for the Supreme Court is whether the decision to prorogue parliament is “justiciable” – whether it is a matter that the courts can examine at all. The High Court’s judgment was that it isn’t; in our system of separated powers, ruled the three senior judges, the judiciary must stay out of “political” or “high policy” disputes. The Prime Minister’s decision to prorogue parliament, and particularly the decision to prorogue for a lengthy period, was “inherently political in nature”, it found, adding that “there are no legal standards against which to judge their legitimacy”. The court refused to rule on the reasons for the prorogation, or whether the Prime Minister had therefore been truthful in his justification of it, but it held that this was not a matter that could be examined by the courts.

In Scotland, however, the Inner Court of Session took a different view. Its starting point was that the decision to prorogue was not one of politics or high policy, but the avoidance of democratic scrutiny. It was designed, it found, to “stymie Parliamentary scrutiny of Government action”. Such scrutiny is "a central pillar of the good governance principle which is enshrined in the constitution”. This was not, the court was careful to note, a matter of Scottish law. It was the application of common law, informed by the principles of “democracy and the rule of law”. Parliamentary debate is central to our constitution; since the Prime Minister had prorogued parliament in order to restrict the time it had to debate Brexit, this was held to be an unlawful use of prerogative powers.

Now the matter arrives at the UK Supreme Court. The context has fundamentally changed since Irvine’s warning in 1996. In 1998, parliament gave the judiciary the task of adjudicating upon human rights issues through the Human Rights Act. Although this brought the UK in line with most developed democracies, and continued a trend which had begun with the evolution of “judicial review” in the 1970s, it predictably led to an increasing number of clashes between judges, politicians and the media. But the courts have taken their new responsibilities seriously, and developed a rich body of UK-focused human rights law. In 2009, recognising that the highest appeal court could not comfortably operate in a separation of powers system whilst adjudicating in a cramped House of Lords committee room and pronouncing judgments in the Lords’ debating chamber, the UK Supreme Court was established. It moved into new premises across Parliament Square and opened itself up to the public as no UK court had previously. Its hearings are now filmed and broadcast live online, an innovation even the US Supreme Court cannot match.

The law rarely works quickly, but nobody watching the Supreme Court since its launch a decade ago can have missed its steady evolution into a constitutional court. Every year or so a ruling has emerged, often on an esoteric subject, in which the justices have gradually cemented the court’s place in the UK’s constitutional structure, at the same time developing a theory of the UK’s constitution, and the court’s powers arising from it, which it then applied in future cases.

For example, in 2014 the court ruled that the government could not override a court’s ruling that Prince Charles’ ‘black spider’ letters were to be released under freedom of information laws. The Supreme Court president said that laws allowing government ministers to overrule court decisions simply because they didn’t agree “would cut across … constitutional principles which are … fundamental components of the rule of law”.

It was through a Supreme Court judgment over HS2 in 2014 that Lord Sumption developed the idea of the UK’s “constitutional instruments” and principles that are “fundamental to the rule of law”, despite the lack of a written constitution. It was the same “principle of legality” which led to the first Gina Miller judgment in 2017, in which the Supreme Court decided that the executive branch could not take the UK out of the EU without parliamentary approval.

Then, a few months later, the court decided in the Unison case that another constitutional principle inherent to the rule of law, access to justice, was breached by the charging of exorbitant fees to bring employment tribunal claims.

The language in these cases is often of lofty principle and ancient law. But in essence, when the court invokes the “rule of law”, the “principle of legality” and “the unwritten constitution”, it is staking a territorial claim. It is saying that the area being discussed is on constitutional land, and is therefore guarded by the courts.

It is against this background that the prorogation case arrives at the Supreme Court. If the court decides that it has no jurisdiction – that the matter is inherently political and therefore “non justiciable” – it would go against the grain of the court’s evolving image of its own role in the constitutional landscape. It would also be, in my view, a dereliction of responsibility.

It has been reported that the Prime Minister may prorogue parliament again in the last two weeks of October, to guarantee Brexit by any means. Whether or not this is true, it highlights the danger that if prorogation for the purpose of avoiding parliamentary scrutiny is permitted by the courts, it will be the thin end of the wedge. The executive would hold a dangerous new power to insulate itself from parliamentary scrutiny.

For the Supreme Court to rule that the Prime Minister has acted in breach of the UK’s constitution would be a defence of the constitutional principles it has been developing for years. But there will be consequences for the court which we, as a liberal democracy, must not shy away from. It is inherently unstable for a constitutional court to operate without a written constitution.

The dangers are clear to see from this case. The judiciary will be accused of politicisation, whichever way it rules. If the court rules against the Prime Minister it will be said, with some justification, that the unwritten constitution gives the judiciary too much discretion - that by merely pronouncing a matter to be ‘constitutional’ it becomes so, and vice-versa. The risk is that deciding this boundary (as opposed to whether facts of individual cases have breached it) becomes a hostage to the personal politics of individual judges.

This can be seen by the approach of Lord Sumption, who has been publicly vocal in his criticism of the Prime Minister’s decision to prorogue, but who recently used his Reith Lectures to warn that the courts are becoming politicised because parliament has asked them to rule on human rights issues. Critics of Sumption’s position, including myself, argued that the line he draws privileges those with power - who are naturally concerned with lofty principles such as the rule of law and free speech - over those without, for example groups who have traditionally fallen outside of the courts’ protections, and who greatly benefited from human rights laws, such as the LGBT community. Ideally, the broad ambit of constitutional protections should be set by democratic consensus, not the views of individual politicians or judges.

As each new constitutional crisis now leads swiftly to litigation, and the mobilisation of metaphorical tanks on opposing lawns, it is surely in everyone’s interests to pause and take stock of the direction our constitutional path is leading us. Ultimately, in a separation of powers system, strong fences make for good neighbours, and a clear constitution is a strong barrier against abuses of power. Irvine was right about this being a “delicate balance”, although in the last two decades the balance has thankfully shifted towards greater scrutiny of the executive. I hope that the Supreme Court does what is right. But if it does, the case for a written constitution will become ever stronger.

Adam Wagner is a barrister at Doughty Street Chambers and the founder and chair of RightsInfo. Follow him at @adamwagner1