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The Liberal Democrats' review into alternatives for Britain's Trident nuclear deterrent concluded that Britain can maintain a strong nuclear stance without a like-for-like replacement of its Trident submarine fleet, but there are still other options to be pursued.
The review said: "The analysis has shown that there are alternatives to Trident that would enable the UK to be capable of inflicting significant damage such that most potential adversaries around the world would be deterred.
"It also shows that there are alternative non-continuous postures that could be adopted, including by SSBNs, (nuclear missile submarines) which would aim to be at reduced readiness only when the UK assesses the threat of a no-notice pre-emptive attack to be low."
The party concluded today that Britain can maintain a strong nuclear stance without a like-for-like replacement of its Trident submarine fleet, but maintained there are other options.
In a statement, Mr Clegg said: "This is the most thorough review of our nuclear deterrent this country has ever published.
"It clearly shows that there are options for our country that do not simply involve us sticking to the same strategic positions that were taken in the Cold War."
"I hope that today marks the beginning of a fact-based debate about Trident that will see us discussing what kind of deterrent is right for Britain in the 21st Century, rather than just sticking to decisions that were made for another time."
The review, led by Treasury Chief Secretary Danny Alexander, said there are alternatives to the UK's current nuclear stance which requires at least one nuclear-armed submarine always to be at a sea.
However, it accepts that cutting the size of the current four-vessel fleet would not offer the same degree of resilience as the current continuous-at-sea deterrent and would not guarantee "a prompt response in all circumstances".
In a move which could put his party at odds with its Conservative coalition partners, the Chief Secretary to the Treasury admitted Britain faces a "big decision in 2016" over alternatives to the current weapon system.
Lord Ashdown has dismissed Defence Secretary Philip Hammond's criticism of downgrading Trident in light of a new review unveiling alternatives to the nuclear deterrent.
The former Liberal Democrat leader wrote on Twitter:
He added that reducing the size of the fleet would not mean a "part-time" deterrent:
Downgrading the UK's nuclear deterrent would be a "huge gamble with Britain's security", Defence Secretary Philip Hammond has declared as he rejected proposals for a "part-time" Trident force ahead of the publication of a Liberal Democrat review of the weapons system.
Mr Hammond insisted that a like-for-like replacement of all four nuclear-armed submarines when they leave service is the only way to maintain a continuous state of readiness to launch a strike.
Liberal Democrat Cabinet minister Danny Alexander's Trident Alternatives Review, which will be published today, is expected to include an array of options short of full replacement.
But Mr Hammond insisted in an interview with BBC Radio 4's Today programme that any financial savings from reducing the fleet would be "trivial".
"We have had for 45 years now a continuous-at-sea deterrent posture which has served this country very well and we do not believe that with nuclear threats.
"If anything, proliferating, with more countries seeking to get nuclear weapons, this is the time to downgrade, certainly not to go to a part-time deterrent."
Prime Minister David Cameron has left no doubt of his preference for like-for-like replacement of the ageing Trident fleet, which was also backed by Labour's Tony Blair in a 2006 review when he was Prime Minister.
A leaked version of Danny Alexander's review suggested that building just two new subs instead of four would save £5 billion in upfront expense and a further £1billion in running costs.
Other options thought to be put forward in his report include a proposal to cut costs by sometimes putting subs to sea without warheads on board.
A group of former defence secretaries and military chiefs have voiced their support for a £25 billion replacement of Britain's nuclear deterrent system, ahead of a long-awaited report which is expected to argue the case for a scaled-down Trident force.
A letter to the Daily Telegraph, signed by five former defence secretaries and two former chiefs of defence staff, warned the Government not to "take risks with our security" by downgrading Trident.
"In an uncertain world, in which the number of nuclear weapons remains high and some states are increasing their holding, we should not take risks with our security by downgrading to a part-time deterrent," they wrote.
Liberal Democrat Cabinet minister Danny Alexander's Trident Alternatives Review is expected to include an array of options short of like-for-like replacement, including a proposal that the Royal Navy's fleet of nuclear-armed submarines could be cut from four to two.
The review of alternatives results from a compromise reached by Conservatives and Liberal Democrats in the 2010 coalition agreement which brought them into government together, and is likely to set a clear dividing line between the parties at the next general election.
Mr Alexander has said he hoped today's review would "open up a debate about the fact that there are different ways of approaching nuclear deterrents."
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