Britain could still maintain a credible nuclear deterrent while cutting back its Trident submarine force, according to a new analysis.

A report published by the Royal United Services Institute (RUSI) military think tank argues the UK does not need to have at least one nuclear missile submarine always at sea to be sure of deterring a nuclear attack.

Ending the so-called continuous-at-sea-deterrent (CASD) would mean the Royal Navy's Trident submarine fleet could be scaled back saving billions of pounds, the paper said.

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The Conservatives are currently committed to a like-for-like replacement for the existing four-boat fleet needed to maintain round-the-clock patrols - at an estimated cost of £20 billion - if they win the next general election, while Labour also supports CASD.

However, after a review of the options for renewing the deterrent last year, the Liberal Democrats said in the post-Cold War era, CASD was no longer necessary and they would only build three new submarines.

The policy was branded "naive" by Defence Secretary Philip Hammond who accused the LibDems of taking a "huge gamble" with national security.

However the paper, by RUSI analyst Hugh Chalmers, said that even if there was not always a submarine on patrol, it would still represent a powerful deterrent to an aggressor.

"Despite criticism that non-continuous postures would create a 'part-time deterrent', even an inactive fleet of submarines can help to deter actors from seriously threatening the UK," it said.