THE Liberal Democrats have called for a historic scaling-back of the Trident nuclear deterrent, despite warnings it could put the UK at risk and claims it would save just £50 million a year.

Danny Alexander, the Chief Secretary to the Treasury, said it would be wrong to blindly continue with a system he denounced as the last bastion of the Cold War.

Announcing the results of a Coalition review into alternatives to Trident, he called for an end to one of deterrent's key principles – 24-hour patrols.

But his party appeared isolated on the issue as Labour joined the Conservatives to reject any end to the principle of a continuous at-sea deterrent.

SNP leader Alex Salmond also attacked the LibDems for a "meek announcement" that still committed the party to spending billions replacing the ageing nuclear weapons system.

Mr Alexander said a move to part-time patrols plus a cut in the submarine fleet from four boats to three would save £4 billion on the estimated £20bn-£25bn cost over decades.

But the Conservatives poured scorn on a "part-time deterrent".

Downing Street said David Cameron remained committed to keeping a submarine at sea at all times.

Labour said it had seen nothing in the review to alter its support for a round-the-clock deterrent.

Jim Murphy, Labour's Shadow Defence Secretary, said: "It would require a substantial body of evidence for us to change that, but this review does not appear to offer such evidence."

Conservative Defence Secretary Philip Hammond also claimed the move from four boats to three would save only around £50m-£60m a year in today's money.

The review was a hard-won concession to the LibDems during its Coalition negotiations with the Conservatives in 2010. But the Liberals refused to say Trident would be a "red line" in any future Coalition negotiations.

The final decision on how to replace the aging system is not due until 2016. All parties are expected to set out their plans in their manifestoes for the next general election, due in 2015.

The LibDems, who concede they are unlikely to enter government except as part of a coalition, had hoped the review would persuade Labour to back a "climb down the nuclear ladder".

Nick Clegg's aides urged Labour to consider seriously the outcomes of the report.

They also said there could be an opportunity for the LibDems if they go into the next election as the only party pledging to cut back the UK's nuclear deterrent.

The report was scathing about alternatives to Trident, including land-based missiles, warning they would be more expensive than a like-for-like replacement.

It did find a credible deterrent could be maintained without continuous-at-sea patrols but officials warned that this would carry with it a greater degree of risk.

Mr Alexander acknowledged the move would mean accepting "different calculation of risk" but said he believed it was justified.

He said: "If continuous-at-sea deterrence is an insurance policy, we're paying too high a premium for our needs."