US-UK Mutual Defence Agreement

CND strongly opposes the little-known nuclear agreement between the United Kingdom and United States of America that is due to be renewed in 2024.

The Agreement between the UK and the USA for Cooperation in the Uses of Atomic Energy for Mutual Defence Purposes – known as the MDA – was agreed in secret by the UK and US in 1959. It allows for the transfer of information relating to nuclear technology and US-UK collaboration over their nuclear weapons programmes. The MDA is essential to the replacement of the Trident nuclear weapons system, yet there is no effective parliamentary or public scrutiny of the agreement. Government assumes that its renewal will be automatically accepted. Yet at this dangerous time in world affairs, renewal of the MDA will perpetuate Britain’s nuclear arsenal and reinforce its failure to comply with the nuclear Non-Proliferation Treaty by disarming its nuclear weapons. It’s time for us to challenge the MDA.

The MDA was last renewed for another ten years in 2014, through a formal process without scrutiny or vote. CND maintains that the terms of the MDA are in contravention of the UK’s legal commitment as signatory to the nuclear Non-Proliferation Treaty (NPT). We demand a full debate on the treaty before it is renewed this time.


The UK government’s claim that its Trident nuclear weapons system is independent is false. It is both technically and politically dependent on the United States, largely due to the MDA, signed by both countries in 1958. The agreement enabled both countries to exchange classified information to develop their respective nuclear weapon systems.

Originally, the MDA prohibited the transfer of nuclear weapons, but an amendment in 1959 allowed for the transfer of nuclear materials and equipment between both countries up to a certain deadline. The MDA as a whole is not time limited, but the amendment that allows transfer of nuclear materials has to be extended through ratification by both parties every ten years, most recently in 2014.

Violation of NPT

The relationship and activities which are enshrined in the MDA confirm an indefinite commitment by the US and UK to collaborate on nuclear weapons technology and violate both countries’ obligations as signatories to the NPT.

The NPT states that countries should undertake ‘to pursue negotiations in good faith on effective measures relating to… nuclear disarmament’. Rather than working together to get rid of their nuclear weapons, the UK and US are collaborating on further advancing their respective nuclear arsenals. A 2004 legal advice paper by Rabinder Singh QC and Professor Christine Chinkin concluded that it is ‘strongly arguable that the renewal of the Mutual Defence Agreement is in breach of the nuclear Non-Proliferation Treaty’ as it implies ‘continuation and indeed enhancement of the nuclear programme, not progress towards its discontinuation’.

NPT signatories are also committed not to transfer any nuclear weapons or explosive devices to any recipient, an action which is core to the functioning of the MDA and is the specific provision that requires the amendment to be renewed every ten years.


Prior to the Constitutional Reform and Governance Act 2010, Parliament was powerless to oppose renewal, but since then, both Houses of Parliament have the opportunity to oppose ratification should they so wish, and the House of Commons has the potential to block the treaty indefinitely.

The government is now required to publish a treaty that is subject to ratification, and lay it before Parliament for 21 sitting days, the so-called Ponsonby rule. The text should be sent to relevant select committees and any requests for debates should be considered favourably. But there is no statutory requirement for the Government to hold a debate or vote, and Parliament cannot amend treaties.

During the 21 sitting days, both Houses have the opportunity to pass a resolution that the treaty should not be ratified. If neither does so, the government can go ahead and ratify the treaty. If either House votes against ratification, the government has to lay a statement giving its reasons for wanting to proceed. If the Commons has voted against ratification, laying this statement triggers a further 21 sitting day period before ratification. This process can be repeated, potentially blocking a treaty indefinitely. If only the House of Lords votes against ratification, then a ministerial statement explaining why the treaty should be ratified is sufficient prior to ratification.

In June 2014, anticipating that year’s renewal, Jeremy Corbyn MP tabled EDM 153 calling for a debate on the MDA renewal to be held in government time. The amendments to the MDA were subsequently laid on 16 October and several MPs called for a Parliamentary debate. On 10 September Jeremy Corbyn argued in the House: “The mutual defence agreement between Britain and the USA on the sharing of nuclear information, originally signed in 1958, comes up for renewal this year. There is no date set for Parliament to debate it, and apparently the Government do not seem terribly keen on that, yet President Obama sent a message to Congress on 24 July saying that he approved of the renewal of the agreement and hoped that Congress would approve it. If it is good enough for Congress to debate the mutual defence agreement, surely it is good enough for us to debate it as well.”

However, as the Act indicates, the government is not obliged to hold a debate or vote, and the onus is on MPs to pass resolutions opposing the renewal. This year – 2024 – with the MDA coming back to parliament for renewal, it is crucial that the full opportunities presented by the 2010 Act be used.

In particular, the government should be made to answer why they are contravening their legal obligation to work towards disarmament and instead renewing an agreement with the US that is designed to maintain both countries’ nuclear weapons production capabilities. The government should be held accountable to international law.

Reliance on US

As a consequence of the MDA, the UK relies on the US for many aspects of Trident. The current UK warhead is a copy of the US one, with some components directly bought from the US. A replacement warhead programme for Britain’s nuclear weapons system will run parallel to the USA’s new W93 Trident warhead development programme.
The UK leases from the US the Trident II D5 missiles it uses and British submarines must regularly visit the US base in Kings Bay, Georgia, for the maintenance and replacement of these missiles. The UK government has paid the US £250 million to participate in a missile life extension programme.

The UK participates in numerous exchange visits with staff from the US nuclear weapons laboratories. It also participates with the US in ‘sub-critical’ nuclear tests (tests which fall just short of releasing a nuclear explosion).


The MDA treaty is intended to facilitate the development of Britain’s nuclear weapon technology and support building a replacement for Trident. This is in direct contradiction to Britain’s legal obligation to disarm.

The treaty also raises a politically sensitive point. By having such a direct involvement in Britain’s nuclear weapons technology, the US exercises significant leverage over the UK’s foreign and defence policy.

It’s time that the government is forced to justify why it continues to insist on having a secret nuclear relationship with the US rather than work with the international community to get rid of nuclear weapons.