Nuclear deterrent 'must be renewed'
The UK should retain and renew its nuclear deterrent, an independent review has concluded.
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.
However it added that the country should show keen regard for the shared responsibility to work towards global nuclear disarmament.
It also said 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 Government will decide whether to renew Trident in 2016.
The Conservatives are committed to a like-for-like replacement for the existing four-boat 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 United Kingdom 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 ongoing 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."
It 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 SSBN (ballistic missile submarines) system meets the criteria of "credibility, scale, survivability, reach and readiness".
The current plans to construct and deploy four replacement SSBN submarines with missiles and warheads over the period 2016 to 2062 will account for 9.4% of the defence budget, the report said.
However it said 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% 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.
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.
Its report also called on the Ministry of Defence to consider what steps it could take to work towards multilateral disarmament.
It said: "Such steps might not only be further reductions in warheads or changes in posture and declaratory policy, but could also include further transparency and verification measures, treaty-based commitments to control and reduce stocks of fissile materials and their means of production, and refraining from certain forms of development or modernisation.
"This will require a more explicit articulation of the conditions necessary for the UK to have the confidence to take such steps, and of national and collaborative actions that could bring these about."
It acknowledged that the UK has adopted a " stance of greater transparency" since the end of the Cold War.
Vernon Coaker MP, Labour's shadow defence secretary, said: "Labour is committed to ensuring Britain retains a minimum credible independent nuclear deterrent, which we believe is best delivered through a continuous at sea deterrent.
"We will look closely at the Commission's findings and welcome its acknowledgement that in the current international security climate it is in the UK's national interest to retain a nuclear deterrent.
"Between now and the Main Gate decision that will be taken in 2016, we will continue to examine ways in which this can be delivered most efficiently, based on expert military advice, capability and cost."
Admiral Sir George Zambellas, head of the Royal Navy, called for four boats, which would delivery "100% availability".
Delivering a keynote speech at the Rusi (Royal United Services Institutes) International Sea Power Conference 2014, Admiral Zambellas, who is First Sea Lord and chief of the naval staff, said Britain needs two aircraft carriers, not one, if it is to provide a credible answer to the global threat to maritime security.
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% 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."
Deputy First Minister Nicola Sturgeon said: "The Scottish Government position is that Trident should be removed from an independent Scotland by 2020. We will also propose a constitutional prohibition on nuclear weapons being based in Scotland, ensuring they would never return.
"September's referendum is an opportunity to remove Trident from Scotland before we are hit with a share of the further £100 billion in costs estimated for the lifetime of the UK Government's proposed successor system for a new generation of weapons of mass destruction .
"That will mean significant savings through the estimated £170 million that Scottish taxpayers contribute every year to the maintenance and running of Trident. That is money which could and will be far better spent on other priorities - something underlined today by statistics showing one million people in Scotland are living in relative poverty."