The Attorney General wrote to the Prime Minister setting out his legal opinion on the Joint Instrument and Unilateral Declaration concerning the Withdrawal Agreement. (12 March 2019)
Dear Prime Minister,
Legal Opinion on Joint Instrument and Unilateral Declaration concerning the Withdrawal Agreement
1. I have considered the documents entitled “Instrument Relating To The Agreement On The Withdrawal Of The United Kingdom Of Great Britain And Northern Ireland From The European Union And The European Atomic Energy Community“(the “Joint Instrument“) and the “Declaration By Her Majesty’s Government Of The United Kingdom Of Great Britain And Northern Ireland Concerning The Northern Ireland Protocol” (the “Unilateral Declaration“), which were concluded with the EU late last night.
2. As the preamble states, the Joint Instrument is an instrument relating to the Withdrawal Agreement which was made in connection with its conclusion and is accepted by both parties. Therefore, pursuant to the principle set out in Article 31 of the Vienna Convention on the Law of Treaties, which is widely accepted as reflecting customary international law, the Joint Instrument has binding legal effect as a document that is to be taken as an authentic interpretation of the Withdrawal Agreement.
3. The Unilateral Declaration sets out the United Kingdom’s position on the Withdrawal Agreement and the Joint Instrument. A unilateral declaration by one party to a bilateral agreement constitutes an authentic interpretation of the treaty if it is accepted by the other party. While it is not an agreed document, it too has legal status as an interpretative document, and will be lodged with the depositary of the Withdrawal Agreement and form part of the context of the treaty in accordance with the principle set out in Article 31 of the Vienna Convention.