As the UK's constitution continues to be tested, the Brexit fundamentals remain unchanged.

Seven weeks from now, the UK will automatically exit the EU on 31 October by the default operation of the Article 50 process. The Government alone need do nothing to make that happen; and Parliament cannot stop it. But Brexit continues to test political and constitutional boundaries, with real implications for what happens next.

1. Parliament has been "prorogued". That means that it will not sit again until mid-October and, importantly, that all outstanding Bills, including measures intended to address Brexit day-one issues, have fallen and will need to start their parliamentary passage afresh. It looks highly unlikely that all of the measures that Government considered necessary to provide for the immediate post-Brexit position can now be introduced by 31 October.

2. Or has it? The highest Court in Scotland has ruled that the prorogation was unlawful and therefore a nullity. In contrast, in England and Wales, a Divisional Court concluded that the lawfulness of the prorogation cannot be judicially reviewed. So, the current position is that the prorogation is lawful and effective in English law, but unlawful and ineffective in Scottish law – both heading to the Supreme Court on appeal next week. If the Supreme Court agrees with the Scottish court that the decision to prorogue can in principle be reviewed for lawfulness, then it will have to consider whether it also agrees that, on the evidence (or, rather, the lack of it from the Government), the Prime Minister's dominant purpose was – contrary to the advice he gave the Queen – to deny the proper role of Parliament in scrutinising Government. If it does, then Parliament may return immediately – although it would not in theory preclude the Government from then seeking prorogation on a proper basis. That conclusion would undoubtedly test the boundaries of judicial review of prerogative decisions but, with no hard and fast exclusion from review, the Supreme Court may well determine that it is no more desirable for it to have to draw the line between "high politics" and justiciability than for it to have to adjudicate, as in this case, on the evidence. That evidence may be augmented in coming days by information published in response to Dominic Grieve's successful motion for a "humble address" requiring the Government to publish details of internal communications regarding Operation Yellowhammer and the prorogation decision. [The government published its initial response to the motion on 11 September 2019.]

3. The Government must seek an extension. Before Parliament was prorogued, it passed an Opposition Bill requiring the Government to seek an extension if it has not by 19 October either reached an agreement with the EU that is acceptable to Parliament or has obtained Parliament's approval to leave without a deal.

4. Or must it? While the Government has — seemingly reluctantly — confirmed that it will abide by the law, the Prime Minister has said that he will not seek an extension. This raises the prospect of his resignation and/or a General Election. The question for the Government will be how it can trigger a General Election without Opposition support, when that support will not be given unless and until the Government has obtained a Brexit extension. Neither of the legal mechanisms for calling an election can be triggered while Parliament is prorogued, as they require a vote of the House of Commons.

5. And if the UK does ask… it will be up to the EU27 to decide whether to extend again – and they must agree any extension unanimously.

So, all eyes on the Supreme Court next week: its decision could see Parliament return, the Prime Minister resign, and an election brought forward. That may or may not affect the Brexit timetable. All is speculation at present. In the meantime, the default position remains a no-deal departure on 31 October, probably without all of the legislative measures that the Government had planned. Businesses and their advisors must prepare for that outcome.