It is my understanding that the legal advice issued by government lawyers (which includes a categorical statement that the action proposed by the British government would be a breach of international law “in a very specific and limited way”) does not make reference to Article 27 of the Vienna Convention on the law of treaties (1969) to which the UK was a signatory.   This states that: “A party may not invoke the provisions of its internal law as justification for its failure to perform a treaty. This rule is without prejudice to Article 46.”

The key wording of Article 27 is contained in the first sentence. However, for the sake of completeness, Article 46 states as follows:  “1. A State may not invoke the fact that its consent to be bound by a treaty has been expressed in violation of a provision of its internal law regarding competence to conclude treaties as invalidating its consent unless that violation was manifest and concerned a rule of its internal law of fundamental importance.2. A violation is manifest if it would be objectively evident to any State conducting itself in the matter in accordance with normal practice and in good faith.”

One other point is also relevant: Article 26 “PACTA SUNT SERVANDA”: “Every treaty in force is binding upon the parties to it and must be performed by them in good faith.”

The question then arises of whether the Ministerial Code requires minsters to act in accordance with international and domestic law.  Government lawyers differ on this.  However, The Court of Appeal has stated that the Ministerial Code (2010): subsumes within the stated duty of minister to “comply with the law” reference to international law and treaty obligations.  “That duty includes those obligations.  Whatever the precise meaning of the reference to those obligations, they are not independent obligations but simply part of the overarching duty of compliance with the law.” R (Gulf Centre for Human Rights v. The Prime Minister and The Chancellor of the Duchy of Lancaster (2018).

If further clarification were required (which in my opinion it was not) in 2015 the Cabinet Office indicated that the phrase “comply with the law” included international law.

It is worth noting that one, very eminent, lawyer has described the Attorney General’s”opinion” which runs contrary to the above as “utterly risible.”

It is also worth noting that newly appointed Cabinet Secretary has opined that the Ministerial Code requires only that ministers adhere to domestic law.  That opinion may be wrong – in which case his tenure of office may be among the shortest on record.