At 10:30 a.m. Tuesday (that’s London time; 5:30 a.m. on the U.S. East Coast), Britain’s highest court opens three days of televised hearings on Brexit in a historic constitutional test of the 700-year-old UK Parliament’s role as the most powerful entity in that country’s government.

The fundamental issue at the core of 12 hours of scheduled public argument is this: will the bold, sometimes defiant new Prime Minister, Boris Johnson, emerge as the truly dominant government figure in today’s Britain, or will he humbled by being put more strictly under the thumb of a newly energetic Parliament?

Behind the legal dispute is whether Brexit – the formal separation of the UK from the European Union after 50 years – will actually happen on October 31, and whether, if it happens, it will be smooth or chaotic.  There is as yet no formal agreement on the terms of the divorce, and most analysts expect that, without such a deal, both the UK and Europe will face serious disruptions across social, economic and political fronts.

Johnson, however, has vowed to carry out the separation on October 31, “do or die,” with or without a deal with the EU.

If the fundamental constitutional dispute were not enough by itself to entice the world to watch three days of high (but decorous) courtroom drama, there it the prospect that – if Johnson wins — the United Kingdom may begin to break up into three or maybe even four separate, independent nations (England, Scotland, Northern Ireland, Wales).

And for those fascinated by the role of courts in a nation’s governing, the UK Supreme Court will be facing a test of that young court’s own self-confidence and creativity.  There appears to be little doubt among many legal experts in Britain that this will be their highest court’s most important decision since its founding in October 2009, split off from the House of Lords to become somewhat independent of Parliament.

The court will sit this week as an 11-Justice tribunal, presided over by Lady Brenda M. Hale, Baroness of Richmond.  Its hearings are scheduled to run Tuesday through Thursday, two sessions each day, with arguments by eight advocates.

The hearings can be watched at this link: https://www.supremecourt.uk/live/court-01.html

Two cases are to be heard, both focusing on whether Prime Minister Johnson acted unconstitutionally last month in moving to suspend the sitting of Parliament for five weeks during a crucial legislative session leading up to the now-scheduled “Exit Day” at the end of next month.

Two lower courts split on that question.  The High Court in London ruled that the decision to send Parliament home (“prorogue” it) was a matter solely for politics, with no role for the courts to play.  The Court of Session of Scotland, in Edinburgh, ruled that Johnson acted unconstitutionally, finding that he had the invalid purpose of keeping Parliament from interfering with his plan to carry out Brexit October 31.

The UK Supreme Court will be under significant pressure to rule without delay.   Under a new law passed this month by Parliament during the brief interval before it was sent home, it ordered Johnson to ask the European Union by October 19 for more time to work out a departure settlement, if there is not one in place and approved by Parliament.

Johnson has said he would never ask for more time, and would refuse an extension even if the EU were to offer one.

There is another prominent legal dispute that will be unfolding this week and next in a court in Scotland and, while it is less prominent than the review by the Supreme Court, it has the potential of resulting in a court order to seek an extension of the Brexit deadline if Johnson refuses to seek or accept one.  (Scottish law provides an unusual procedure, nobile officium, allowing its courts to order government officials to carry out a duty if there is no other institution that could do so.)

One of the more unusual facets of both this separate dispute and the two cases before the Supreme Court is how far the courts of the UK are bound to follow Scottish law that dates back to the 17^th Century.  Scotland’s ancient laws have been significantly more protective of Parliament than are the laws of England.

That explains, in significant part, why the Scottish Court of Session ruled against Johnson on the prorogation issue.  One of the tasks confronting Johnson’s challengers as they argue this week in the Supreme Court is to persuade those Justices to fully embrace Scottish law in deciding this controversy.

If the Supreme Court does rule against Johnson, on whatever legal or constitutional reasoning, it presumably would do no more than rule – as the Scottish court did – that the prorogation was invalid.  It would then be up to Parliament to decide, on its own, whether it wants to go back into session to take further action to prevent a departure without a deal with the EU.

If, however, it rules that the courts should have stayed out of the feud between Parliament and Johnson, the Prime Minister would appear to be free to go ahead and plan for a no-deal Brexit – unless, of course, the Scottish court handling the other dispute would take steps to prevent that from happening.