It is a well-established principle of international law that the state is obligated to fulfill its treaty obligations in good faith. This is and will remain the guiding principle in the UK’s approach to international relations. However, in the extremely difficult and exceptional circumstances in which we find ourselves, it is important to remember the basic principle of parliamentary supremacy.
Parliament is sovereign in terms of domestic law and can pass legislation that violates the UK’s treaty obligations. Parliament will not act unconstitutionally in enacting such legislation. This “two-way” approach is shared by other similar legal systems such as Canada, Australia and New Zealand. Under this approach, treaty obligations become binding only to the extent required by domestic legislation. Whether the enactment of legislation or its repeal, the content of that legislation is for Parliament and Parliament alone. This principle was recently approved unanimously by the Supreme Court in R (Miller) v. Secretary of State for Exiting the European Union  UKSC 5.
Legislation implementing the Withdrawal Agreement, including the Northern Ireland Protocol, is expressly governed by the principle of parliamentary sovereignty. Parliament’s ability to pass provisions that would take precedence over the withdrawal agreement is explicitly affirmed in Section 38 of the European Union (Withdrawal Agreement) Act of 2020, with specific reference to the EU law concept of “direct effect”.
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