*Bold
Also from the comments, a judge’s argument.
If this is right, then the actions of HMG, through the Permanent Representative, cannot bind either Parliament or the UK generally on the international law plane so that any purported extension of the Article 50 period would not be valid. At the very least, HMG has acted in a manner which flouts an Act of Parliament and proper parliamentary procedure.
How should it have been done? There should have been non-binding negotiations with the EU on the new “exit date”. The draft SI should then have been laid before both Houses using the Sch 7 para 14 procedure, with the new proposed date(s) in it. If approved the EU should have been told and it could then make a formal decision. Then the Minister could sign the SI.
The way the government has acted, by trying to force Parliament to agree to what it has done unilaterally without prior parliamentary approval, is, at the least, highly unsatisfactory. It must be arguable that the government has acted illegally.
Rt Hon Sir Richard Aikens, is a former member of the Court of Appeal, and former Vice-President of the Consultative Council of European Judges.
http://tapnewswire.com/2019/03/judge-declares-brexit-delay-illegal/