Treaties - Notes for Students

Numbers at the end of paragraphs refer to the reading links at the end of this post.

International Law, in general, may be described as "the body of legal rules, norms, and standards that apply between sovereign States and other entities that are legally recognized as international actors."  (1)

A key aspect of international law relates to TREATIES

which are, international agreements concluded between States in written form and governed by international law. They can comprise a single instrument or there may be two or more related instruments. They also have various names - notably Treaty, Convention etc. (2)

International Law has to be distinguished from the domestic law of individual States. For the UK, that requires international law to be distinguished from the distinct domestic legal jurisdictions - principally of England and Wales, Scotland, Northern Ireland.

The UK's Dualist System

When the UK enters into a treaty, the treaty will bind the UK so far as international law is concerned but it does not affect the law within the UK unless there is an Act of Parliament applying it. This is known as a "Dualist" system. (3)

The UK's Treaty-Making (and unmaking) power

Within the UK, the power to make treaties rests with the Crown and is known as a Royal Prerogative Power. In practice, it is the government of the day (i.e. Ministers of the Crown) who negotiate and enter into treaties on behalf of the UK. If a treaty requires change to domestic law then it is for Ministers to bring legislative proposals to Parliament.

The word "ratification" refers to the formal act by which a State consents to be bound by a treaty. The negotiating partners sign the treaty text but that does not usually bring the treaty into force because most treaties require a formal act expressing consent to be bound.

In 2010, the Constitutional Reform and Governance Act placed Parliamentary scrutiny of treaty ratification on a statutory footing. In particular, the Act gives legal effect to a resolution of the House of Commons that a treaty should not be ratified. This means that should the House of Commons take the view that the Government should not proceed to ratify a treaty, it can resolve against ratification and thus make it unlawful for the Government to ratify the treaty.

In addition to the CRAG procedure, where a treaty requires domestic legislation for the UK to comply with the obligations under the treaty, it is government practice to insist that implementing legislation must be passed before ratification.

The role of Parliament regarding treaties is therefore less extensive than one might at first think. If legislation is required to change domestic law then Parliament has a much greater role - e.g. by the need to enact legislation. (4)

A greater role for Parliament?

The treaty-making power is remarkable in that it enables Ministers to enter into all manner of arrangements with foreign States and such arrangements may have very serious and long-term, perhaps unforeseen, implications. The low level of parliamentary scrutiny may therefore be seen as a weakness in the UK's constitutional practices though it has to be remembered that legislation may be required to implement a treaty if it is to have legal effect domestically. 

Similarly, it is remarkable that Ministers are able to renounce treaties provided that they are willing to run the risks of any international consequences for the UK if they do. Again, legislation may be required to alter domestic law. When the UK left the European Union, legislation was enacted to repeal the European Communities Act 1972.

There are those who wish to see a much greater role for Parliament in treaty-making though it appears unlikely that Ministers will wish to readily give up the prerogative treaty-making power. (5)

Reading Links

1) On international law in general see, for example, House of Commons Library Research Briefing 21 September 2020, Encyclopedia Britannica - International Law,  Council on Foreign Relations - What is International Law?

2) Vienna Convention on the Law of Treaties 1969

3) Treaties usually specify when they will enter into force. Dualism and Monism are explained at Peace and Justice Initiative - Resources. See also UCL Reforming the Prerogative.

4)  Royal Prerogative powers are discussed in this 2017 House of Commons Research Briefing and also at Taming the Prerogative (House of Commons 2004). See also

CRAG 2010 Part 2 - Readers of the legislation will very soon see that the power given to Parliament by the 2010 Act is limited - e.g. it does not apply to certain types of treaty etc.

5) A "must study" topic for law students has to be the convoluted legislative process for BREXIT. In particular, the Supreme Court's judgment in R (Miller) v Secretary of State for Exiting the EU [2017] UKSC 5. Numerous posts on this blog and elsewhere examined this topic.

The second "Miller case" dealt with the power to prorogue Parliament - HERE.

International Agreements Committee - House of Lords - see Review of Treaty Scrutiny. 

The International Agreements Committee (IAC) is the principal body in Parliament tasked with scrutinising treaties under the framework established by the Constitutional Reform and Governance Act 2010 (CRAG). Since its establishment in 2020, the IAC has issued two reports on working practices and has made recommendations to Government on how to make treaty scrutiny more effective within the CRAG framework. 

House of Commons Research Briefing - How Parliament treats reaties

Joshua Rozenberg - Scrutinising Treaties (May 2025)