Boris Johnson’s entire Brexit strategy has rested on the assumption that Britain’s unwritten constitution gives him the right to drive a cart and horses through parliamentary process and convention. Yesterday’s British Supreme Court decision blew a hole through the Prime Minister’s plan.

The 11 judges ruled unanimously that Johnson’s hugely controversial decision to advise the Queen to suspend Parliament — called prorogation — for five weeks was judged to be “unlawful, void and of no effect”.

Real forks in history are probably few and far between, but surely this was one of them. Whatever the implications for Brexit itself — and we’ll come to that — the ruling yesterday is a victory for parliamentary democracy and the rule of law in a country where faith in both has been shaken badly.

For the first time, the court has decided to test the exercise of prerogative power by the Government on the matter of prorogation.

Had the court decided it had no jurisdiction, or that the suspension was lawful, a prime minister could have disbanded Parliament at any time to avoid scrutiny. Instead, after forensically examining the Government’s evidence, the court found it wanting.

Brenda Hale, the Supreme Court’s president, coolly laid out the findings in the manner of a longstanding headteacher explaining the application of school rules in a disciplinary hearing.

The case before the court, Hale noted, was ultimately about the limits of the Prime Minister’s power to advise the Queen to prorogue Parliament for five weeks.

The court ruled that “no reasonable justification” for that length of suspension was put forward — the underlying reason was no doubt Johnson’s desire to sideline Parliament so it didn’t interfere with his strategy to deliver Brexit “come what may” on Hallowe’en.

In the absence of such justification, the court said, parliamentary sovereignty would be undermined if the executive prevented Parliament from making law and carrying out its functions.

Most damning, the court ruled that “the effect upon the fundamentals of our democracy was extreme”. It is hard to imagine stronger words to denounce the Johnson government’s modus operandi.

The court left it to House of Commons Speaker John Bercow and his counterpart in the House of Lords to determine what to do next. Bercow was ready: he immediately released a statement saying that the Commons must “convene without delay”.

The court made clear as it heard the case that it wasn’t ruling on the timing or manner of Brexit itself, but its bombshell judgment cannot but affect the debate about how, when or even whether Brexit happens.

At a minimum, the decision should place renewed pressure on Johnson to pursue a deal with the European Union, although Brussels officials maintain that no serious proposals have been put forward.

However, the ramifications are likely to go farther. Johnson sought to suspend Parliament so it couldn’t remove his threat of a no-deal Brexit; instead, as the opposition Labour Party’s Brexit spokesman, Keir Starmer, said on Monday night, the Johnson gambit galvanised opponents of “no deal” across parties and resulted in legislation that forces the Government to seek an extension to the October 31 departure date.

Johnson also gambled on being able to call an early election. That, too, was rebuffed.

He fired 21 lawmakers for voting against the Government, making any agreement on his strategy even more difficult.

In other times, the head of government would resign after such a powerful rebuke. Misleading the Queen and the repeated failure of his high-risk strategy in Parliament would be other reasons to go.

Johnson swatted away such an idea, but there are calls for the head of his chief adviser and strategist, Dominic Cummings. That one of these calls came from Brexit Party leader Nigel Farage shows how worried Brexiteers are that the miscalculation may jeopardise the whole project.

One shouldn’t doubt Johnson’s ability to demonise his opponents or change tack towards a more fruitful path to hold on to a job he has always coveted.

But the Prime Minister, who is due to meet Donald Trump today in New York, has clearly been weakened. Unlike the US Supreme Court, the younger British court is considered entirely apolitical.

Meanwhile, a Labour Party haplessly divided over its own Brexit policy has been handed a significant fillip.

Delegates at the party’s conference in Brighton erupted in jubilation when the decision was announced.

If you thought the House of Commons was gladiatorial before, just wait until it reconvenes. Labour leader Jeremy Corbyn, who called for the Prime Minister to resign, may now seek to table a vote of no confidence. Johnson would do well to get on a flight back to London.

He must now decide whether he will double down on his “people versus the establishment” campaign or pivot to something else.

From that perspective, the most important aspect of the decision was its unanimity: if the Government wants to paint this as an elitist stitch-up, it must campaign against the very idea of having a Supreme Court and the notion of a law-based society.

Johnson has been pulling his party away from core conservative principles, but this would be utter self-destruction. The Government’s lawyers had left open the possibility of a new prorogation, so that can’t be discounted.

The court didn’t rule on Johnson’s motives — that is, it didn’t actually call him a liar — but it did say his stated reasons for the suspension were not sufficient. So any repeat will have to meet a higher bar set by the court.

In the meantime, Parliament will have time to shift the focus back to the Commons, where Johnson will face ferocious scrutiny and surely more doubters in his own fracturing party.

He likened himself recently to the Hulk, the Marvel Comics superhero who develops superhuman strength and a green hue when angry.

His lawyers had warned the court about “very serious practical consequences” if it found against the Prime Minister. No doubt, he’s mad now. But the legal, political and civil service establishment that he so vilifies has developed its own superpowers.

• Therese Raphael writes editorials on European politics and economics for Bloomberg Opinion. She was editorial page editor of The Wall Street Journal Europe