I’ve finally found something authoritative on the use of a Statutory Instrument (as opposed to a new Act of Parliament) to change the Brexit date. It’s from the Hansard Society blog. It’s all very complicated, but the short version seems to be that as long as all goes to plan and Parliament votes for it then the government should be able to get it through in time.
However, given the steps outlined above, the process could be accelerated. We see no insuperable procedural obstacle to proceedings on the ‘exit day’ SI being completed by 29 March if the draft SI were laid, for example, on Friday 22 or Monday 25 March.
But there are still possible banana skins, for example, if the EU cannot agree with May until after Monday 25th over when the exit date should be. Barnier, for instance, is warning May that there may be no extension without ‘more clarity’ (yes, that old one), but you don’t know how much of this is just theatre, or to give May an excuse to go for a long delay to kill off Brexit.
Another possible banana skin is this:
Interesting speculation that A50 extension might need primary legislation as it relates to previously retained Crown prerogative powers impacted in the Miller judgement at the Supreme Court in January 2017. I'm sure legal eagles are checking it out?
— Stewart Jackson (@BrexitStewart) March 19, 2019
Getting a full Act, rather than an SI, through Parliament in five-six days seems unlikely, to say the least. It took six days just for the original European Union (Withdrawal) Act (EWA) to go from being approved by Parliament to getting Royal Assent and becoming law.
Anyway, legal action concerning this issue could possibly derail the process for long enough to prevent the SI getting through, although it seems a long shot at the moment.
Interestingly, the Hansard Society post notes that the EWA did allow for an SI to be used to adjust the date. That doesn’t mean for sure that an SI will do the job, though, as wider issues (such as those alluded to by Jackson) may mean that an Act is required. But it is interesting to note that this was an amendment to the Act made in response to machinations by one O. Letwin:
at committee stage during passage of the Bill in December 2017, the government successfully moved two amendments: …
one, moved as a compromise after an initiative from the government backbencher Sir Oliver Letwin MP, to give ministers a delegated power to use an SI to amend the definition of ‘exit day’ in the Act, “to ensure that the day and time specified in the definition are the day and time that the Treaties are to cease to apply to the United Kingdom” if “the day or time on or at which the Treaties are to cease to apply to the United Kingdom in accordance with Article 50(3) of the Treaty on European Union is different” from the definition of ‘exit day’ in the Act (i.e. 29 March at 11pm) (amendment 400, agreed without division, now Section 20(4) of the Act).
What a far-sighted and cunning Remainer that O. Letwin was.