Affirming a lower court decision, the UK Supreme Court has held that, despite the referendum in June 2016 calling for withdrawal from the European Union, Britain cannot withdraw from the Union without parliamentary approval. In doing so, the Court emphasized the “constitutional character” of the legislation that implements the UK’s membership in the EU (para. 67).
In the UK, the prerogative powers of the executive branch include the authority to make and withdraw from treaties. Treaties operate as domestic law in the UK, however, only when implemented by statute, and only Parliament can make, repeal, or modify statutory law. In 1972, shortly before the UK ratified the accession treaty to join what is now the EU, Parliament enacted the European Communities Act.
The Court explained that the 1972 Act not only gives domestic effect to EU treaty provisions, but also “authorises a dynamic process by which, without further primary legislation (and, in some cases, even without any domestic legislation), EU law not only becomes a source of UK law, but actually takes precedence over all domestic sources of UK law, including statutes.” (para. 60) Pursuant to the Act, “the EU Treaties, EU legislation and the interpretations placed on these instruments by the Court of Justice [of the European Union] are direct sources of UK law.” (para. 61)
In light of these features, withdrawal from the EU, the Court explained, will result in “a fundamental change in the constitutional arrangements of the United Kingdom.” (para. 78) The Court said that it could not “accept that a major change to UK constitutional arrangements can be achieved by ministers alone; it must be affected in the only way that the UK constitution recognises, namely by Parliamentary legislation.” (para. 82)
The Court also noted that withdrawal from the EU would result in a loss to UK citizens of various rights and remedies conferred under EU law, and it reasoned that such loss was “another, albeit related, ground” for its conclusion. (para. 83) The Court elaborated:
[T]he EU Treaties not only concern the international relations of the United Kingdom, they are a source of domestic law, and they are a source of domestic legal rights many of which are inextricably linked with domestic law from other sources. Accordingly, the Royal prerogative to make and unmake treaties, which operates wholly on the international plane, cannot be exercised in relation to the EU Treaties, at least in the absence of domestic sanction in appropriate statutory form (para. 86).
What next? Prior to the decision, the UK prime minister, Theresa May, had indicated that if the Court ruled this way, she would quickly seek to have Parliament enact a statute authorizing withdrawal. Whether the process for obtaining such authorization will delay the prime minister’s filing of the formal notice of withdrawal required by Article 50 of the Treaty on European Union remains to be seen. The Supreme Court’s suggestion that the executive branch may not be able to withdraw the UK from treaties that confer domestic “rights and remedies” will also likely be invoked in debates over whether the UK should denounce the European Convention on Human Rights—a goal of some Brexit supporters.
It may reasonably be asked whether the analysis in the UK Supreme Court’s decision is relevant to debates in the United States over whether the President has unilateral treaty withdrawal authority, especially in light of President Trump’s statements that he might seek to withdraw the United States from certain treaties. (His announcement yesterday indicating that the U.S. would not join the TPP is different, since the U.S. was never a party to that treaty.) Any such relevance is likely to be limited, for at least two reasons.
First, while modern U.S. practice suggests that the President, like the executive branch in the UK, probably has a unilateral power to withdraw from treaties, in other respects the two countries’ constitutional law relating to treaties differs substantially. For example, unlike in the UK, presidents are constitutionally required to obtain legislative approval for many international agreements (either through the consent of two-thirds of the Senate or a majority of both houses of Congress). In addition, treaties in the United States, unlike in the UK, can be “self-executing” and thus have direct domestic effect. Furthermore, it is not clear whether and to what extent Congress can restrict the President’s authority to withdraw from treaties, whereas the UK Supreme Court assumed that Parliament had the authority to restrict executive branch withdrawal.
Second, much of the reasoning of the UK Supreme Court is about the constitutional character of the EU treaty regime as implemented by Parliament, including an expansive delegation of authority to EU institutions and the ongoing incorporation of EU law of various types into the UK legal system. The United States is not a party to anything comparable.
That said, there is a difficult question in the United States about how the President’s unilateral authority to withdraw from treaties (assuming the President has this authority) operates with respect to treaties that have been implemented by statute. As in the UK, the executive branch in the United States has no authority to make or unmake statutes. In theory, therefore, legislation that implements a treaty could continue in effect even after a president withdrew the United States from the treaty. Whether it would do so would presumably turn on whether Congress intended the statute’s continuing operation to be conditioned on the United States remaining a party to the treaty. But this is a somewhat different issue from whether an implementing statute should be construed as disallowing executive branch withdrawal from the treaty—the issue addressed by the UK Supreme Court.