Chapter 4 – Protect Our People 35 ambiguous about precisely when, how and at what scale we would contemplate their use, in order not to simplify the calculations of any potential aggressor. 4.69 The UK will not use, or threaten to use, nuclear weapons against any Non-Nuclear Weapons State party to the Treaty on the Non-Proliferation of Nuclear Weapons (NPT). This assurance does not apply to any state in material breach of those non-proliferation obligations. While there is currently no direct threat to the UK or its vital interests from states developing weapons of mass destruction, such as chemical and biological capabilities, we reserve the right to review this assurance if the future threat, development or proliferation of these weapons make it necessary. Working with NATO, the US and France 4.70 Nuclear deterrence is an important part of NATO’s overall strategy. Since 1962, the UK has declared our nuclear capability to the defence of the Alliance, thereby contributing to the ultimate guarantee of collective Euro-Atlantic security. 4.71 The UK works closely with the US and France on nuclear matters, including nuclear policy. UK and US nuclear defence cooperation is underpinned by the recently renewed 1958 Mutual Defence Agreement and the 1963 Polaris Sales Agreement. Among other things, these allow the UK to reduce costs by procuring Trident missiles and other components from the US while maintaining full operational independence. We collaborate with France under the 2010 Teutates Treaty to develop the technologies associated with the safe and effective maintenance of our respective nuclear stockpiles. Replacement warheads 4.72 Work continues to determine the optimum life of the UK’s existing nuclear warhead stockpile and the range of replacement options. A replacement warhead is not required until at least the late 2030s, possibly later. Given lead times, however, a decision on replacing the warhead may be required in this Parliament or early in the next. In the meantime, we continue to invest significantly in the Atomic Weapons Establishment to maintain the facilities and skills necessary to assure the safety and security of the current stockpile, and to sustain the ability to develop a replacement warhead when we need to do so. The Successor submarine programme 4.73 We will replace the Vanguard Class of nuclear-armed submarines with a new class of four submarines, currently known as Successor. This is a national endeavour, and is one of the largest government investment programmes, equivalent in scale to Crossrail or High Speed 2. 4.74 In 2011 we started the design phase. Since then, the MOD has worked with its main industrial partners – BAE Systems, Rolls-Royce and Babcock – to deliver the submarine programme. Improvements are required to deliver this national endeavour, drawing on the experience of major projects in government and the private sector in the UK and beyond. We will create a world-class, enduring submarine enterprise. This will require sustained long‑term effort, but we are committed to doing so and will be taking a number of important steps in this Parliament. 4.75 Working with industry, we will confirm and implement the organisational, managerial and contractual changes needed to deliver the Successor programme. To do so, we will: