Lawyers acting for activists and politicians who have brought this case argue Johnson sent the M.P.s away deliberately to "silence" them and stop them scrutinizing his hard-line Brexit plans at a momentous time in the country’s history.
Some even questioned whether Johnson lied to the Queen about his true intentions for suspending the government when he had to ask for her permission as is mandated by an archaic quirk of Britain's constitutional monarchy.
For his part, the prime minister has been adamant his decision to suspend Parliament was legitimate and nothing out of the ordinary, claiming it was necessary to prepare for a new parliamentary session and new legislative agenda.
Johnson’s government faced several legal challenges to his suspension of Parliament but because they produced conflicting rulings, the matter has gone up to the Supreme Court.
Last week, Scotland’s highest court of appeal ruled that Johnson’s decision to suspend Parliament for five weeks during the ongoing Brexit crisis was unlawful "because its purpose was to stymie parliamentary scrutiny" of the government.
In another case earlier this month, London’s High Court rejected a legal challenge against the decision, but allowed it to be taken to the Supreme Court for an appeal.
In a hearing that begins Tuesday and is scheduled for three days, the Supreme Court must decide if the suspension is a matter for the courts, and, if that is the case, whether Johnson broke the law.
The proceedings that will be streamed live online are expected to include submissions from the governments of Scotland and Wales as well as former Prime Minister John Major.
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The court is not expected to rule immediately, but a judgment could come at the end of this week or early next week.
If the government wins, the Parliament will stay suspended until Oct. 14.
But should the government lose, then the court, at least in theory, could order M.P.s to return to Parliament.
Asked by the BBC what he will do if the Supreme Court rules against the government, Johnson dodged the question Tuesday, only saying he will "wait and see what the judges say."
More than three years after the U.K. voted to leave the European Union, exactly how and when the country will make its departure remains up in the air with options ranging from a last-minute divorce deal, an abrupt exit with no deal and abandoning the whole endeavor altogether.
Johnson has been steadfast in his promise to take the country out of the bloc on Oct. 31, even comparing himself to the Incredible Hulk in his determination to meet the fast-approaching deadline. He has also said he would rather be "dead in a ditch" that ask for another extension from the E.U.
The majority of Britain’s lawmakers want to avoid a “no-deal” scenario, alleging it would be ruinous for the country’s economy.
Earlier this month, they passed a bill that would block a no-deal exit and force Johnson to seek another extension to the Oct. 31 deadline unless parliament has either approved a deal or consented to leave without one by Oct. 19.
They also refused Johnson’s request to hold a snap election to let British voters decide how they want Brexit to play out.
But time is running out for the embattled prime minister to secure a divorce deal with the E.U. — if he is ever to get one.
As recently as Sunday, Johnson said he "passionately believed" that he could strike a Brexit deal within weeks.
The prime minister has also insisted he's been making “huge amount of progress” in negotiations with the bloc, but many are skeptical about it, including the E.U. itself.
On Monday, Johnson went to Luxembourg — the second smallest E.U. member state — to meet European Commission President Jean-Claude Juncker, who later said in a statement that it's up to the U.K. to present a workable solution to the Irish border issue, one of the most contentious points in the negotiations, but such proposals have not yet been made.
After the meeting, the PM was booed by protesters outside.
The crowd and the booing intensified when he later met with Luxembourg Prime Minister Xavier Bettel.
Johnson was due to address journalists alongside Bettel in the courtyard after their meeting, but left straight away as the heckling continued. Instead, Bettel spoke with reporters beside an empty podium intended for Johnson, with the Union Jack still in place.
He urged the British prime minister to offer more than “just words” when it comes to negotiations and provide clarity to the Europeans living in the U.K.
“So now it’s on Mr. Johnson,” Bettel said, pointing to the empty podium.
“He holds the future of all the U.K. citizens and every E.U. citizen living in the U.K. in his hands. It's his responsibility. Your people, our people count on you. But the clock is ticking - use your time wisely."