Could the Withdrawal Agreement be terminated under international law?

The Leigh amendment & the Vienna Convention on the Law of Treaties

An amendment from Sir Edward Leigh to the Government’s motion in December 2018 on approving the Withdrawal Agreement called for an assurance from the Government that it would terminate the Withdrawal Agreement if the EU refused to agree to removing the backstop by the end of 2021. The Leigh amendment did not pass, but the debate about the possibility of terminating the Withdrawal Agreement resurfaced after the Prime Minister secured clarifications and interpretations concerning the Irish Protocol in March 2019.

But could the UK terminate the Withdrawal Agreement under provisions in the Vienna Convention on the Law of Treaties (VCLT)? The VCLT sets out limited circumstances in which a treaty can be denounced when it lacks relevant clauses on denunciation and when the parties to the treaty cannot be brought into agreement. These circumstances include cases where:

• the parties clearly intended to admit the possibility of denunciation, or a right of denunciation is implied by the nature of the treaty;
• a further treaty covering the same subject matter is concluded by all the parties to the existing treaty and supersedes this treaty, or the two are incompatible;
• there is a material breach of a bilateral treaty by one party, or in certain circumstances by a party to a multilateral treaty;
• it is impossible for a party to observe a treaty because of an essential treaty objective disappearing;
• there has been a fundamental change in the circumstances under which the parties originally concluded the treaty, and that change in circumstances has not resulted from the actions of the state wishing to denounce;
• a new peremptory norm of general international law has emerged with which the treaty conflicts.Strictly speaking, the Leigh amendment does not claim that the Vienna Convention applies, but it implies that it does. At the very least, the Vienna Convention is a very strong guide to the international law rules - and it works as a guide even for the EU (which is not a party to it), as confirmed by the CJEU in its Wightman judgment on 10 December 2018.

Does the Vienna Convention apply?

The Withdrawal Agreement is a treaty between the UK (a state) and the EU (an international organisation), whereas ‘treaty’ is defined in the VCLT as an agreement between states; and the Vienna Convention involving international organisations is not yet in force.

Strictly speaking, the Leigh amendment did not claim that the Vienna Convention applied, but it implied that it did. At the very least, the Vienna Convention is a very strong guide to the international law rules - and it works as a guide even for the EU (which is not a party to it), as confirmed by the Court of Justice of the EU (CJEU) in its Wightman judgment on 10 December 2018.

'Material breach' and 'fundamental change of circumstances'

Some commentators have suggested that either the ‘material breach’ (Article 60) or ‘fundamental change of circumstances’ (Article 62) provisions in the VCLT could allow the UK to unilaterally denounce or terminate the agreement if the Irish backstop were to become permanent. But there are difficulties with these suggestions. Proving a ‘material breach’ of 'best endeavours' would be extremely difficult; and crucially, a continuation of the backstop would not be a 'fundamental change of circumstances' within the meaning of the VCLT. It is foreseen in the Protocol; so are possible adverse effects of such a continuation and a mechanism is provided in the Protocol to tackle such a situation.

Dispute resolution in the Withdrawal Agreement

Any dispute about the Withdrawal Agreement will have to be settled within the Withdrawal Agreement dispute settlement mechanisms (eg, Joint Committee settlement or arbitration) or by subsequent international agreements between the parties to it. Within those dispute settlement mechanisms, the parties are able to reference customary international law to justify their position, including that codified by Articles 60 and 62 of the VCLT.