34 National Security Strategy and Strategic Defence and Security Review The MOD’s current basing footprint does not meet the needs of our modern Armed Forces. We will make the MOD’s estate better suited, while reducing it and making it more affordable. We will invest in key defence sites, and reduce the built estate by 30%, releasing public sector land for 55,000 new homes to support wider prosperity objectives. C. The nuclear deterrent The role of the nuclear deterrent 4.63 The UK’s independent nuclear deterrent will remain essential to our security today, and for as long as the global security situation demands. It has existed for over 60 years to deter the most extreme threats to our national security and way of life, helping to guarantee our security, and that of our allies. Since 1969, the Royal Navy has delivered the nuclear deterrent under Operation Relentless, with at least one of four nuclear-armed submarines on patrol at all times. 4.64 Other states continue to have nuclear arsenals and there is a continuing risk of further proliferation of nuclear weapons. There is a risk that states might use their nuclear capability to threaten us, try to constrain our decision making in a crisis or sponsor nuclear terrorism. Recent changes in the international security context remind us that we cannot relax our guard. We cannot rule out further shifts which would put us, or our NATO Allies, under grave threat. 4.65 The Vanguard Class of nuclear-armed submarines will begin to leave service by the early 2030s. The time it will take to design, develop and manufacture its replacement means that the decisions that we take now will affect our security into the 2050s. It would be irresponsible to assume that the UK will not in the foreseeable future be confronted with the kinds of extreme threat to our security or way of life which nuclear weapons seek to deter. We judge that a minimum, credible, independent nuclear deterrent, based on Continuous At Sea Deterrence and assigned to the defence of NATO, remains vital to our national security. We will therefore make the necessary investment to sustain our Continuous At Sea Deterrence. The UK’s minimum, assured, credible nuclear deterrent 4.66 We are committed to maintaining the minimum amount of destructive power needed to deter any aggressor. This requires us to ensure that our deterrent is not vulnerable to pre-emptive action by potential adversaries. Our assessment, after considering the alternatives, remains that four submarines are needed, in order to give assurance that at least one will always be at sea, undetected, on a Continuous At Sea Deterrent patrol. Submarines on patrol will continue to carry 40 nuclear warheads and no more than eight operational missiles. We will retain no more than 120 operationally available warheads and, by the mid2020s, we will reduce the overall nuclear weapon stockpile to no more than 180 warheads, meeting the commitments set out in the 2010 SDSR. 4.67 We will continue to keep our nuclear posture under constant review in the light of the international security environment and the actions of potential adversaries. UK nuclear weapons policy 4.68 Only the Prime Minister can authorise the launch of nuclear weapons, which ensures that political control is maintained at all times. We would use our nuclear weapons only in extreme circumstances of self-defence, including the defence of our NATO Allies. While our resolve and capability to do so if necessary is beyond doubt, we will remain deliberately