Amendments to the UK-US Mutual Defence Agreement

Authors note: The amendments to the Mutual Defence Agreement entered into force on 14 November 2024.

For more than 65 years the UK and the United States have cooperated on defence nuclear issues.

The Agreement between the UK and the USA for Cooperation in the Uses of Atomic Energy for Mutual Defence Purposes 1958, also known as the Mutual Defence Agreement (MDA), allows the United States and the UK to exchange nuclear materials, technology and information. It does not provide for the transfer of nuclear weapons or control over such weapons. It was the result of an amendment to post- second world war US non-proliferation law, which exempted allies that had made substantial progress in developing nuclear weapons from the general ban on exchanges that might lead to nuclear proliferation.

While the MDA as a whole is not time limited, one of the most important parts of the treaty which allows for the exchange of nuclear materials and non-nuclear components (Article III bis) is time limited and is due to expire at the end of 2024. An amendment to the treaty extending this deadline must therefore be ratified by both States Parties and brought into force by 31 December 2024.

Proposed treaty changes

The Government published its amendments to the MDA as Command Paper 1135 on 25 July 2024.

Many of the proposed changes are minor technical amendments relating to outdated language, definitions and terminology and to reflect the different classifications of information that may be exchanged under the MDA. There are three amendments, however, which are worthy of note:

• Article 4 which makes the provisions on naval nuclear propulsion cooperation reciprocal and allows the UK to transfer technology to, and share information with, the US.
• Article 5 which removes the expiry provisions that relate to article III bis and allows for the MDA, as a whole, to remain in force on an “enduring basis”. As such, the agreement will not require renewal every ten years.
• Article 13 adds new final provisions to the agreement that will ensure that information, material or equipment shared or transferred under the MDA will continue to be protected should the agreement be terminated by either party in the future.

The treaty amendments come at a time when the international security environment is becoming more volatile and the UK, and the US, continue to modernise their strategic nuclear deterrents. 

Parliamentary scrutiny

Under the Constitutional Reform and Governance Act 2010 both Houses can oppose ratification should they so wish, but only the House of Commons has the theoretical potential to block the treaty indefinitely. If neither House passes a resolution opposing ratification within 21 sitting days, the Government can go ahead and ratify the treaty. The deadline for passing a resolution opposing ratification is 23 October 2024.

Criticisms of the MDA

Critics argue that continuation of the MDA contravenes the parties’ disarmament obligations under the Treaty on the Non-Proliferation of Nuclear Weapons 1968 (NPT). Others have suggested that it perpetuates a cycle of dependence that the UK has on the United States with respect to nuclear weapons.