The UK will have stepped back from its obligations under the nuclear non-proliferation treaty if it goes ahead with plans to increase its nuclear arsenal, the civil servant who was in charge of the Foreign Office until last autumn has warned. The surprise conclusion by Lord McDonald, the former permanent secretary at the Foreign Office, has been backed by two academics at the London School of Economics who were commissioned by the Campaign for Nuclear Disarmament (CND) to examine Boris Johnson’s pledge. McDonald, seen as a cautious civil servant, told the House of Lords: “I understand that a continuous at-sea deterrent needs us to be able to deploy two boats from time to time. The new ceiling allows both boats to be fully armed. But that does not increase deterrence. It is expensive and incompatible with our obligations under the nuclear non-proliferation treaty. “In 1968, the non-nuclear-weapon states accepted that as their permanent status in exchange for two things: the sharing of the benefits of peaceful nuclear technology, and that nuclear-weapon states would work towards nuclear disarmament. The government assert that the objective is untouched, but the announcement is a step away from its achievement.” The CND opinion was led by Prof Christine Chinkin, a longtime consultant for the UN, and Dr Louise Arimatsu, a former fellow at the Nato Cooperative Cyber Defence Centre. They found the proposed increase constituted a breach of article six of the non-proliferation treaty (NPT). CND is considering seeking a judicial review to declare that the UK’s decision set out in the integrated foreign and defence review breaches UK obligations. The integrated defence review said the UK would “move to an overall nuclear weapon stockpile of no more than 260 warheads” and replace its existing nuclear warhead to ensure continued deterrence with modern nuclear systems. The announcement in effect meant an increase in the UK’s overall nuclear weapon stockpile from 180 to 260 warheads, representing a rise of more than 40%. The review justified the measure “in recognition of the evolving security environment, including the developing range of technological and doctrinal threats”. The CND legal opinion states: “The announcement by the UK government of the increase in nuclear warheads and its modernisation of its weapons system constitutes a breach of the NPT article VI.” That article places an obligation on signatories to pursue disarmament negotiations in good faith. The opinion points out the UN secretary general’s office has come to the same conclusion. The opinion adds that the UK would be in breach of international law were it to use or threaten to use nuclear weapons against a state party to the NPT solely on the basis of a material breach of the latter’s non-proliferation obligations. It also finds the UK would be in breach of international law were it to use or threaten to use nuclear weapons in self-defence solely on the grounds that the future threat of weapons of mass destruction, such as chemical and biological capabilities or emerging technologies, could have comparable impact to nuclear weapons. Foreign Office ministers in the past have always upheld the importance of the NPT, reflecting a view inside the department that is evidently not shared in the Cabinet Office, or the Ministry of Defence. When asked about the NPT in the Commons in November last year, the Foreign Office minister James Cleverly said: “The United Kingdom’s priority is to reinforce the non-proliferation treaty.” The MoD said: “Maintaining the UK’s nuclear deterrent capability at a minimum credible level is fully consistent with our international legal obligations, including those under article VI of the non-proliferation treaty. “The UK’s independent nuclear deterrent exists to deter the most extreme threats to the UK and our Nato allies. Our stockpile ceiling is a maximum if required, not a target nor our current number, and is kept under review.” The ministry stressed that so long as the UK maintained a deterrent it will work for the preservation and strengthening of effective arms control, disarmament, and non-proliferation measures, considering the prevailing security environment.