SNP MP Joanna Cherry and pro-EU businesswoman Gina Miller
HOW WILL IT PLAY OUT?
The case at the highest court in the land will last three days - and follows separate cases against the Government over the Prime Minister's advice to the Queen to "prorogue" Parliament until 14 October - one of the longest periods in modern history.
While the act of prorogation, which tees up a Queen's Speech to introduce a new legislative period, is not controversial in itself, opponents have said the timing in the run-up to the Brexit deadline has been used to shut out MPs.
As the final court of appeal, the Supreme Court panel will decide whether to back the Scottish Court of Sessions's ruling that prorogation was unlawful, or the separate judgment in England that said it was a political issue, and not for the courts to interfere.
WHAT DID THE COURT OF SESSION RULE?
The Inner House of the Court of Session in Edinburgh last week found that the Prime Minister’s advice to the Queen to prorogue parliament was unlawful, following a case brought by 75 MPs.
That came a week after a judge at the Court of Session rejected an attempt by a cross-party group of politicians to have the suspension declared as such.
A Government spokesman said the suspension was “necessary” to allow a new legislative agenda to be proposed.
But the three judges ruled that “its purpose was to stymie parliamentary scrutiny of the executive, which was a central pillar of the good governance principle enshrined in the constitution.”
They said: "The Lord President, Lord Carloway, decided that although advice to Her Majesty the Queen on the exercise of the royal prerogative of prorogating Parliament was not reviewable on the normal grounds of judicial review, it would nevertheless be unlawful if its purpose was to stymie parliamentary scrutiny of the executive, which was a central pillar of the good governance principle enshrined in the constitution; this followed from the principles of democracy and the rule of law."
If the Supreme Court upholds that judgment, the Commons could be forced to reconvene immediately, sparking a fresh round of Parliamentary drama as Boris Johnson faces extra scrutiny Number 10 was not banking on.
Furthermore, if the Court decides that, because it was unlawful, Parliament was never in fact prorogued, the passage of bills from the last session could continue as before.
Raphael Hogarth, an associate at the Institute for Government, said: “If the Supreme Court rules next week that the prorogation was unlawful, then I’d expect Parliament to be sitting again in very short order.
“The mechanics of that depend on what the court says. The court might say that Parliament was never prorogued at all in the eyes of the law and so is actually still sitting after all. Or, the Government might need to recall Parliament immediately.”
WHAT DID THE HIGH COURT FOR ENGLAND AND WALES RULE?
A separate challenge to the prorogation, brought forward by pro-EU activist Gina Miller, was quashed by the High Court of England and Wales earlier this month on the grounds that the issue was “political” and therefore a “non-justiciable exercise of prerogative power” - effectively arguing that the courts did not have a role in sorting this one out.
In their judgment, Lord Chief Justice Lord Burnett, Master of the Rolls Sir Terence Etherton and President of the Queen's Bench Division Dame Victoria Sharp said: "We concluded that the decision of the Prime Minister was not justiciable (capable of challenge). It is not a matter for the courts."
They added: "The Prime Minister's decision that Parliament should be prorogued at the time and for the duration chosen and the advice given to Her Majesty to do so in the present case were political.
"They were inherently political in nature and there are no legal standards against which to judge their legitimacy."
They said it was "impossible for the court to make a legal assessment of whether the duration of the prorogation was excessive by reference to any measure".
Ms Miller’s team had argued that it was an "unlawful abuse of power" and one that breached the legal principle of Parliamentary sovereignty.
Her QC, Lord Pannick, said the PM's decision was "extraordinary" - both because of the "exceptional length" of the suspension and because Parliament would be "silenced" in the crucial weeks up until the deadline.
WHAT HAPPENED IN NORTHERN IRELAND?
Still with us? How about another case just to complicate things?
A judge at the High Court in Belfast last week threw out claims that a no-deal Brexit would breach the Good Friday Agreement.
The case was brought forward by Raymond McCord, a campaigner whose son was killed by loyalist paramilitaries, and two others.
Last week Lord Justice Bernard McCloskey dismissed the argument against prorogation within the case, given it was already at the centre of the cases in England and Scotland.
On that concerning the impact on the peace process, Lord Justice McCloskey said: “I consider the characterisation of the subject matter of these proceedings as inherently and unmistakably political to be beyond plausible dispute.
"Virtually all of the assembled evidence belongs to the world of politics, both national and supra-national."
SO HOW MIGHT THE SUPREME COURT RULE?
The Institute for Government has said that while the Supreme Court will "probably" try to render English and Scottish law as consistent - it could "in theory agree with both the English High Court and the Scottish Court of Session, and rule that the prorogation was lawful under English law but not under Scottish law. In that case, the prorogation would be unlawful in the UK overall."
Ministers have said they will abide by the Supreme Court's ruling - although quite how such an unprecedented constitutional moment will play out is anybody's guess.
Alternatively, if the Government's move is found to be lawful, the prorogation will continue as planned, in what will be seen as a major boost for Number 10.