Mutual Defence Agreement

A sixty year-old nuclear agreement between the United Kingdom and United States of America was last renewed in 2014 with no parliamentary debate or vote. The British public and parliamentarians initially found out about that latest extension and ratification of the Mutual Defence Agreement (MDA) when President Obama informed the United States Congress. CND maintains that the terms of the MDA are in contravention of the UK’s legal commitments as signatories to the nuclear Non-Proliferation Treaty (NPT).


The UK government’s claim that the Trident nuclear weapons system is independent is false.  It is both technically and politically dependent on the United States, largely due to the ‘Agreement between the UK and the USA for cooperation in the Uses of Atomic Energy for Mutual Defence Purposes’, signed by both countries in 1958. Also referred to as the Mutual Defence Agreement (MDA), the treaty established an agreement between 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. This amendment is extended through a renewal of the treaty every ten years, most recently in 2014.

Violation of NPT

The relationship and activities which are enshrined by the MDA confirms an indefinite commitment by the US and UK to collaborate on nuclear weapons technology and violates 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 treaty to be renewed every ten years. Earlier this year, Members of Parliament laid down an Early Day Motion (EDM) in the House of Commons stating that the MDA ‘undermines’ UK and US commitments to the NPT, as its signatories are expected not to transfer any nuclear weapons or explosive devices, and calling for a debate.


The British government is committed by law to lay before Parliament for 21 days any treaty which it has signed and needs to be ratified, the so-called Ponsonby rule. The text should be sent to relevant select committees and any requests for debates should be considered favourably. However, while the US Congress is able to veto the treaty, the British Parliament is unable to object and is not required to ratify the treaty.

Despite parliamentary questions from MPs asking the government about its intention to renew the MDA, it only emerged that the amendment had been extended and ratified when President Obama informed the US Congress. Denying Parliament the opportunity for a discussion, and refusing to disclose the content of the agreement, marks a troubling day for British democracy. This is unacceptable behaviour on such an important issue.

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 for its actions, not be allowed to push through a highly sensitive treaty extension without any debate.

Reliance on US

As a consequence of the MDA, the UK relies on the US for many aspects of Trident. The UK warhead is a copy of the US one, with some components directly bought from the US. With the UK’s nuclear warheads expected to be non-operational by the late 2030s, a decision on their replacement will be intrinsically linked to the work taking place as part of the MDA.

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 recently 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.