Interesting question: if Johnson Government refuses to agree to extend transition, before 30 June 2020 deadline, but later realises / admits just how damaging Hard Brexit will be on 1 January 2021, esp mid-virus crisis, is there any other way to extend transition? Some thoughts:

1) in theory, additional transitional time could be created relatively simply, even after the existing deadline for extension has passed, i.e. by adopting a new EU-UK treaty that amends the existing Withdrawal Agreement, as matter of public international law, to provide new date

2) from a UK constitutional perspective: that new treaty could be agreed by the Government acting under prerogative powers, but would then need to be implemented into UK law by primary legislation… surely a piece of cake for Johnson with his 80 seat Commons majority

3) but from an EU constitutional perspective: any such new treaty could be much more problematic. Remember: despite decades of Europhobic poison dripped by Tory press and politicians, the EU is not a state and has no sovereignty. It is an international organisation. Which means…

4) … for every action it proposes, the EU must identify an authorising provision within its founding treaties: a so-called “legal basis” through which the Member States have consented to the idea of the EU being able to do what it now proposes to do.

5) the existing Treaties contain a great many legal bases, in all sorts of areas, but each is unique, e.g. in terms of its scope of application, the strength of the power conferred, the applicable conditions / limits / exclusions, the relevant decision-making procedures etc...

6) So the key question is: what legal basis could the EU legitimately point to, in order to enter into a new agreement with the UK, amending their existing Withdrawal Treaty, so as to provide for a brand new transition period extension, even after the existing facility lapses?

7) E.g. Withdrawal Agreement itself provides for the Joint Committee to amend parts of the existing treaty under certain circumstances. But it’s difficult to see how that “internal” power of amendment could allow prolongation of transition after explicit extension power lapses

8) And e.g. we can probably safely rule out the EU simply adopting another (amending) treaty under Article 50 TEU itself: the latter’s potential to provide the legal basis for acts in relation to the UK was surely exhausted by the act of withdrawal?

9) Maybe EU could extend UK “status quo” using wide array of specific legal bases scattered throughout Treaties. But if so: no chance of claiming (as happened with Article 50, fully supported by all Member States) that this = "exceptional" power justifying exclusive EU competence

10) Instead: any such "extra transition treaty" would inevitably involve questions of national competence and qualify as a "mixed agreement" – raising possibility of lengthy judicial action to clarify disagreements and / or possibility of national ratifications across/within EU27

11) So: if we despised experts are correct that Hard Brexit on 1 Jan 2021 is undesirable & if Government now thinks experts are worth listening to... life would be much easier if UK uses existing power to extend transition (rather than try to do it later to save political face).

ps, for those interested in the legal nature of Article 50 TEU / withdrawal within the wider system of EU competences: the above notes were drawn from my Common Market Law Review article analysing the UK's Withdrawal Package - due for publication in Issue 3, June 2020.