In the late 1960s, it became apparent to Britain's most senior defence planners that the ability of the United Kingdom's Polaris submarine-launched ballistic missiles to reach their targets in the Soviet Union was threatened by the deployment of a new Soviet anti-ballistic missile (ABM) system around Moscow. After prolonged debate, it was decided in 1970 by the new Conservative Government to begin a feasibility and project definition study for a highly-secret Polaris improvement programme which was designed to give the missiles the capability to penetrate Soviet ABM defences. Having rejected other alternative options, in late 1973 full development of the improved system was approved by ministers. Despite intense budgetary pressures, growing technical problems with the project (leading to major escalation), and anti-nuclear sentiment within the Labour Party, the Labour Government that assumed office in 1974 continued with the project, restricting knowledge of what was now called the Chevaline programme to only a small group of ministers and top officials. When its existence was revealed to the House of Commons in January 1980, immediate attention was drawn to the high costs of the programme (then amounting to about £1 billion), the technical problems it had apparently encountered, and the consequent slips in timescale. In 1982, just a few months before Chevaline was finally deployed, the Public Accounts Committee issued a report which heavily criticised the Ministry of Defence's management of the whole programme, and the way it had been hidden from any form of parliamentary scrutiny since instigation of feasibility and project definition. The Chevaline programme has since become renowned as one of the most controversial aspects of post-war British defence policy. The proposed research will examine the history of the Polaris improvement programme in the context of the development of the British strategic nuclear deterrent from the decision to acquire the Polaris system at the Nassau Conference in December 1962, to the final deployment of Chevaline two decades later. The focus of the research will be on the policymaking of successive British governments as they sought to maintain the credibility of the deterrent, and the problems that were faced by the improvement programme during the 1970s, as timescales slipped and costs escalated. The research will result in the production of a volume in the Cabinet Office's official history series, and will constitute a comprehensive and standard work of reference on the subject, of interest to academics, policymakers, and a more general audience interested in modern British history, defence policy, and the topical issues surrounding Britain's continued possession of a strategic nuclear deterrent.