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Replacing Trident

Although it has reduced its nuclear stockpile since the end of the Cold War, the UK maintains a continuous at-sea nuclear deterrent, now based solely on the submarine-launched Trident system, which consists of four Vanguard-class submarines, Trident II D5 missiles and associated warhead.


The Labour Government committed in 2006 to renewing the UK's nuclear deterrent, and this position was endorsed by the next Government's Strategic Defence and Security Review in 2010. In 2016 decisions will be made on taking the programme forward, including the size of the deterrent fleet, which is expected to enter service from 2028. A decision on any replacement warhead is expected to be made around 2019.

In the context of changing strategic threats and constraints on the overall defence budget, questions will continue to be asked about the rationality and cost-effectiveness of the programme. Calls for greater Parliamentary scrutiny, including a vote on the Main Gate decision in 2016, are likely to dominate this issue going forward. However, it will be for the new Government to decide on how Parliament gets to scrutinise Main Gate and whether there will be a Parliamentary vote.

Policy Background

In 2006 the Labour Government published The Future of the United Kingdom's Nuclear Deterrent, which set out proposals for replacing the UK's nuclear deterrent beyond the 2020s by building a new class of nuclear-powered submarines, carrying the current Trident missile system. The House of Commons voted in March 2007 to support the Government's decisions to "take the steps necessary to maintain the UK's minimum strategic nuclear deterrent beyond the life of the existing system".

The October 2010 Strategic Defence and Security Review endorsed the Labour Government's plans, labelled the 'Successor' programme, whilst also making several changes. The service life of the Vanguard class has been extended, with a view to the first successor submarine entering service in 2028. Continuous at-sea-deterrence will be maintained and a final decision on the size of the submarine fleet will be made at "Main Gate" in 2016. The existing warhead will remain viable until the late 2030s and a decision on any replacement warhead will thus be deferred until 2019.


Chart 1: Procurement cycle

The procurement of defence equipment in the UK is generally conducted according to the CADMID cycle.

diagram showing the main gate


In July 2013 the Liberal Democrats published a review of alternatives for the Successor programme. While the review acknowledged that there are no real alternatives to the current proposals within the timeframe suggested, it argued that the deterrent could be reduced to a contingency posture. This would involve ending continuous nuclear patrols at sea, but maintaining a nuclear capability and exercising it regularly to maintain relevant skills. This proposition is also advocated by many commentators.

Others have argued that an air-launched, as opposed to submarine-launched capability would still provide a credible minimum nuclear deterrent, at less cost. Advocates of unilateral disarmament continue to call for the abandonment of the UK's nuclear deterrent in its entirety.

Progress and costs to date

The submarine is being developed jointly by the Ministry of Defence (MOD), BAE Systems, Rolls-Royce and Babcock International. Approximately 2,200 people are working on the programme. Jobs are expected to peak at 6,000 during the manufacture phase, with approximately 850 British companies involved in the supply chain.

Concept work on the design of the new submarine concluded in May 2011, when the programme's Initial Gate was approved. The project is now in its assessment phase, which seeks to further develop the submarine design and associated costings ahead of the Main Gate decision.

The programme is funded from the MOD's core budget. Forecast costs remain within the estimates set down in the 2006 White Paper: £15–20 billion, including £11–14 billion for the submarine (2006/07 prices). In-service costs are expected to remain at 5–6% of the annual defence budget (i.e. £2bn to £2.5bn based on the budget's current size).

Approved assessment phase spending is £3.3bn. By the end of 2013-14, just over £2 billion had been spent (£854 million on the concept phase and £1.2 billion on the assessment phase). A number of long-lead items for the submarines, including the steel, have already been ordered. The years of peak expenditure will be between 2016/17 and the late 2020s.

Opponents of the nuclear deterrent have suggested that the lifetime costs of the deterrent, including its in-service and decommissioning costs, will amount to more than £100 billion. Questions have been asked about the rationality and value for money of such spending at a time of austerity and further expected cuts in the defence budget. Advocates of the Successor programme argue that the price is comparatively small when compared with the strategic risks involved in renouncing the nuclear deterrent.

Parliamentary Scrutiny

Ahead of the Initial Gate decision, various calls were made for Parliament to have a further vote before the programme proceeded into its assessment phase. Approval of Initial Gate was announced in May 2011, and without a vote in Parliament.

At the time an MOD Minister stated that "Parliament does not routinely review internal Ministry of Defence business cases and I have not yet heard a convincing argument that suggests that this programme should be any different". The MOD did, however, commit to publishing an annual report of progress on the programme.

Calls are likely to continue for greater Parliamentary scrutiny, including a vote on the Main Gate decision in 2016. However, the MOD has been consistent in its view that "It will be for the [new] Government to make decisions about scrutinising the Main Gate decision". While it is broadly accepted that the Main Gate decision will be presented to Parliament, there is no obligation on the Government to give Parliament a vote on whether to take the Successor programme forward.


The UK's disarmament obligations and the legality of replacing Trident

The legality of replacing Trident is hotly contested. Critics argue that by replacing their nuclear forces, the recognised nuclear powers, including the UK, are failing to meet their disarmament obligations under Article VI of the Nuclear Non-Proliferation Treaty (NPT).

Successive Governments have insisted, however, that replacing Trident is compatible with the NPT, arguing that the treaty contains no prohibition on updating existing weapons systems and gives no explicit timeframe for disarmament.

They have also highlighted the steps taken by the UK in support of the NPT, in particular the significant reductions in the British nuclear arsenal since the end of the Cold War (see below). Furthermore they have insisted that sustainable nuclear disarmament can only be achieved through a multilateral process.

Reductions in the UK's nuclear warheads

  • At its Cold War peak the UK nuclear stockpile consisted of approximately 520 nuclear warheads.
  • By 1998 the deterrent had been reduced to one single system: Trident.
  • The 1998 SDR announced a reduction to fewer than 200 operationally available warheads. Total stockpile estimated at 280.
  • The 2006 White Paper planned to reduce the stockpile to fewer than 160 operationally available warheads.
  • In 2010 information on the size of the UK's overall nuclear stockpile (225 warheads) was published for the first time.
  • The 2010 SDSR announced a reduction to 120 operationally available warheads; while the overall stockpile would be no more than 180 by the mid-2020s. Once that reduction has been achieved, the UK nuclear stockpile will have been reduced by 65% since the end of the Cold War.

Party Lines

  • Conservatives: will retain the Trident continuous at sea nuclear deterrent… and build the new fleet of four Successor Ballistic Missile Submarines
  • Greens: decommission the Trident nuclear deterrent system
  • Labour: committed to a minimum, credible, independent nuclear capability, delivered through a Continuous At-Sea Deterrent
  • Liberal Democrats: procure fewer Vanguard successor submarines; move from CASD to a contingency posture of regular patrols
  • SNP: will oppose plans for a new generation of Trident nuclear weapons and will seek to build an alliance in the House of Commons against Trident renewal
  • UKIP: support Trident renewal

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