Britain’s nuclear deterrent should be renewed, according to the conclusions to an independent review.
The cross-party Trident Commission said the UK should retain and deploy a nuclear arsenal for reasons of national security and its responsibilities to Nato, ahead of the Government’s decision over whether to renew Trident in 2016.
However, it added that the country should show keen regard for the shared responsibility to work towards global nuclear disarmament and that relaxing continuous at sea deterrence (CASD), where one submarine is always at sea, could be considered, though it was divided over whether the country could take the step independently or with other nations.
The Conservatives are committed to a like-for-like replacement for the existing four-boat Trident ballistic missile submarine fleet, but the Liberal Democrats say they would only build three new submarines.
Alex Salmond's Scottish Government has made its opposition to nuclear weapons clear, and wants the Trident submarines to be moved out of Scotland in the event of a ‘Yes’ vote in this year's independence referendum.
In its report, the Commission said: "If there is more than a negligible chance that the possession of nuclear weapons might play a decisive future role in the defence of the UK and its allies, in preventing nuclear blackmail, or in affecting the wider security context within which the UK sits, then they should be retained.
"The impact of the UK's falling victim to on-going strategic blackmail or nuclear attack is so significant that, even if the chances appear slim today, there is sufficient uncertainty surrounding the prospect that it would be imprudent to abandon systems that have a high capacity to counter such threats."
The commission considered three credible possibilities where the deterrent effect of an independent British nuclear capability might become decisive.
These were the re-emergence of a nuclear threat from a state with an "aggressive posture"; an existing or emerging nuclear state attaining global reach and entering into direct strategic competition with the UK; and the emergence of a future threat involving bio-weapons or comparable mass destruction technologies.
The report concluded that the Trident system meets the criteria of "credibility, scale, survivability, reach and readiness".
The current plans to construct and deploy four replacement submarines with missiles and warheads over the period 2016 to 2062 will account for 9.4 per cent of the defence budget, the report said, but this should be weighed against the deterrent role of nuclear weapons.
It said: "Over the life of the project, it can be expected that capital, running and decommissioning costs associated with the nuclear weapons project account for roughly 9-10 per cent of the overall defence budget, though into the 2020s we will experience a higher spend, and after that a smaller amount.
"However, we believe that cost must be of secondary importance to the judgement over whether forsaking the UK's nuclear deterrent capability could open the country to future strategic risk."
The Trident Commission was set up in 2011 by the British American Security Information Council and members include former Conservative defence secretary Sir Malcolm Rifkind, ex-Liberal Democrat leader Sir Menzies Campbell, former chief of the defence staff Lord Guthrie of Craigiebank and ex-Labour defence secretary Lord Browne.
Admiral Sir George Zambellas, head of the Royal Navy, called for four boats, which would delivery "100 per cent availability" while delivering a keynote speech at the Royal United Services Institutes’ International Sea Power Conference 2014, where he also called for Britain to have two aircraft carriers rather one.
He said having two carriers ensuring continuous availability was "a modest extra premium to pay" for an "effective, credible, available, insurance policy", and added: "In my book, that same principle of insurance applies to the UK's nuclear deterrent.
"A posture which delivers less than 100 per cent availability is not available, and therefore not credible – not just in the eyes of potential adversaries, but in the eyes of our key strategic partners as well.
"That is why the current UK Government supports a continuous at-sea deterrent, a position supported by our Opposition defence spokesman just last week. In my professional judgment, that means four boats in the new Successor class – to replace our four current V boats."