Errol Mendes is a professor of constitutional and international law at the University of Ottawa, and the president of the International Commission of Jurists, Canada.

In a damning ruling delivered on Tuesday afternoon, the very nature of parliamentary democracy around the world was shaken. The British Supreme Court found that the advice given by Prime Minister Boris Johnson to the Queen – that is, that she should prorogue Parliament – is indeed subject to judicial review.

What this means, fundamentally, is that this most powerful weapon in the hands of a British prime minister is not above the law – that it is “justiciable.” The 11 justices unanimously ruled that the courts can review whether the advice is lawful if it has the purpose or effect of frustrating or preventing Parliament from carrying out its constitutional role without any reasonable justification.

Story continues below advertisement

British Supreme Court rules PM Boris Johnson’s decision to suspend Parliament was illegal

Focusing not on the actions of the Queen but the lawfulness of the advice given to her by the Prime Minister, the court ruled that Mr. Johnson’s recommendation – to bring the British Parliament to a halt for five weeks to bring forward a new throne speech – was deemed unnecessary and that this would typically warrant a suspension of Parliament for four to six days. This unreasonably long pause, in the view of the court, was done in quite exceptional circumstances, given the profound constitutional change due on Oct. 31, which would terminate Britain’s membership in the European Union. In effect, the court declared that Mr. Johnson had lied to the Queen to mask his real purpose: frustrating Parliament’s efforts to prevent a no-deal Brexit.

His advice to the Queen was declared unlawful, void and of no effect. The prorogation was also voided. It is now up to the Speaker to determine what happens next, in terms of bringing Parliament back.

In the view of the court, the order to prorogue Parliament as orchestrated by Mr. Johnson was effectively handing him a blank legislative cheque. In a stunning part of the judgment, the court ruled that the prorogation was not a proceeding in Parliament that prevented the courts from interfering under the 1688 Bill of Rights – but rather that the prorogation had been imposed by actions from outside Parliament and had brought its core and essential business to an end.

The ruling is of profound importance not only in Britain, but to all systems around the world derived from the Mother Parliament, including Canada’s. It establishes that the rule of law applies even to what had been assumed to be a prime minister’s unassailable political power to request the monarch (or her representative, the governor-general, in the case of countries such as Canada) to prorogue Parliament.

When former prime minister Stephen Harper prorogued the Canadian Parliament in 2008 to avoid a no-confidence vote and again in 2009 to stall a parliamentary committee dealing with the Afghan-detainee controversy, most experts concluded that such pursuit of the suspension of Parliament by a prime minister was not subject to judicial review, even when it was clearly designed to avoid parliamentary accountability. Most experts had also accepted that the governor-general, as representative of the Queen, had to accept the advice of the prime minister to prorogue, despite the position-holder’s theoretical reserve power to reject the suggestion.

This historic ruling might change the way we understand the latter issue. If a future Canadian prime minister gives advice to a governor-general to prorogue Parliament, and the purpose was to avoid the accountability of that legislative body without any reasonable justification, the question of whether it could be subject to judicial review is now an open one.

Given that the ruling could well prevent Britain from crashing out of the European Union without a deal in place, it may also signal the beginning of a re-examination of some of the supposedly unassailable powers of prime ministers in parliamentary democracies in Canada and elsewhere. It would be unworkable to amend the Canadian Constitution to reflect this, but reform could happen through restriction of the prime minister’s power to ask for prorogation, rather than seeking to limit the ability of the governor-general to grant it.

Story continues below advertisement

Critical review of our foundational systems isn’t necessarily a bad thing. In fact, as was the case in Britain, Canada’s democracy demands it.

Keep your Opinions sharp and informed. Get the Opinion newsletter. Sign up today.