The court's entry into Brexit stems from PM Boris Johnson's "do or die" promise to take UK out of the EU without any further delay.
The British government and its opponents faced off Tuesday at Britain's Supreme Court in a high-stakes legal drama over Brexit that will determine whether new Prime Minister Boris Johnson broke the law by suspending parliament at a crucial time ahead of Britain's impending departure from the European Union.
In a case that pits the powers of the UK's legislature against those of its executive, the government's opponents argued that Johnson illegally shut down parliament just weeks before the country is due to leave the 28-nation bloc for the "improper purpose" of dodging lawmakers' scrutiny of his Brexit plans.
They also accused Johnson of misleading Queen Elizabeth II, whose formal approval was needed to suspend the legislature.
The government countered that, under Britain's largely unwritten constitution, the suspension was a matter for politicians, not the courts.
Government lawyer Richard Keen said judges in a lower court had "nakedly entered the political arena" by ruling on the matter.
The government is also arguing that parliament is dissolved yearly at the end of every session and that Johnson was simply clearing the way for a new agenda following Theresa May's resignation in July.
Johnson sent lawmakers home on September 9 until October 14, which is barely two weeks before the scheduled October 31 Brexit day.
The prime minister says Britain must leave the EU at the end of next month with or without a divorce deal.
'To silence parliament'
But many UK lawmakers believe a no-deal Brexit would be economically devastating and socially destabilising and are determined to thwart him.
Lawyer David Pannick, who represents one of the campaigners challenging the government, told 11 Supreme Court judges that Johnson had improperly suspended the legislature "to silence parliament ... because he sees parliament as an obstacle to the furtherance of his political aims."
Johnson says the suspension is routine and will allow his government to launch its domestic agenda with a new session of parliament.
But the decision outraged many lawmakers, who say it's designed to prevent them from challenging Johnson's "do or die" push for Brexit in October.
The suspension sparked legal challenges, to which lower courts have given contradictory rulings. England's High Court said the move was a political rather than legal matter, but Scottish court judges ruled last week that Johnson acted illegally "to avoid democratic scrutiny."
The Supreme Court is being asked to decide who was right, in a case scheduled to last up to three days.
The court must now rule on Johnson's motivation, and whether it has the right to make political adjudications at all.
The High Court in England has sided with Johnson while its Scottish counterpart called the suspension "unlawful."
London's Queen Mary University law professor Sionaidh Douglas-Scott said the Supreme Court — the highest legal authority in all of Britain — has not faced a case of this kind since the 1600s.
She said the verdict could have monumental consequences not only for Brexit but also how the world's oldest parliamentary democracy functions down the line.
"If parliament is prorogued [suspended] with no remedy available then the balance of power is tipped far too heavily to the executive," she said.
Former prime minister David Cameron — the Conservative leader who resigned after losing the Brexit referendum in 2016 — said he also thought Johnson was "trying to restrict the debate."
"It looked to me, from the outside, like rather sharp practice of trying to restrict the debate," he said.
A defeat for Johnson would leave him open to charges that he has effectively lied to Queen Elizabeth II, who gave the formal order to suspend parliament.
Britain's beloved monarch is the formal head of state and has little choice but to trust the prime minister when he asks her to end parliamentary sessions.
Fears of crashing out of EU
Johnson is already skirting the edges of the law by insisting that he would rather be "dead in a ditch" than ask his European counterparts to postpone Brexit for a third time.
Parliament has passed a law forcing him to ask the other European leaders for a delay if no compromise emerges from an October 17 and 18 EU summit in Brussels.
Johnson's hand is helped by widespread fears of what a chaotic "no-deal" end to Britain's 46-year involvement in the European project would have on businesses and the public.
The government has been forced to publish documents warning that this outcome could spark civil unrest and shortages of food and medicines.
Small groups of more moderate MPs on both sides of the political divide have reportedly expressed a willingness to back a compromise agreement.
The EU would also prefer to avoid getting blamed for helping push one of its main members out without any safety arrangements.
But its leaders refuse to sign off on any deal that muddies the bloc's trading borders — something they accuse Britain of trying to do.