Once an extension is agreed, domestic law won’t take us out of the EU

As we wait for the text of Theresa May’s letter requesting an Article 50 extension, it is worth remembering that once the UK government and the EU have agreed an extension then the UK will continue to be an EU member under international law—and international law trumps domestic law.

A Department for Exiting the European Union briefing note for ministers points out that the Vienna Convention declares:

”’A party may not invoke the provision of its internal law as justification for its failure to perform a treaty.” As a matter of EU law, it follows that in these circumstances we would remain a member state after 29 March, and the EU law consequences of that would continue to flow in the UK.’

What this means, in layman’s terms, is that even if parliament was prorogued or the statutory instrument trying to change the date was somehow defeated in parliament, the UK would still be regarded as an EU member under both EU and international law.