Labour is "unconvinced" that a scaled-back replacement of Trident would protect national security and save tens of billions of pounds, party sources have made clear.

It follows reports that a long-awaited review will suggest significant savings can be made by ending the practise of keeping the UK's nuclear deterrent constantly at sea.

Labour's cool response suggests Liberal Democrats face an uphill battle to convince the party to join in opposing a like-for-like replacement, the Conservative's preferred option.

Reports suggest the Coalition review could set out potential savings of £5 billion if the UK cuts its submarine fleet from four boats to three or even two.

Extra savings of up to £1 billion a year, on areas like staffing, could also be made, it is expected to say.

But Labour is understood to view those figures as optimistic and believes they do not answer serious questions about national security.

One Labour source rejected suggestions that such an option would pave the way for Liberal-Labour agreement on the issue.

"We have yet to see any concrete, let alone any paving stones," he said.

The LibDems admit that as the smallest of the three parties likely to make up the next UK Government they need to convince either Labour or the Tories.

David Cameron is already on record as saying that he backs a "like-for-like" replacement of the ageing nuclear deterrent on the Clyde.

Yesterday a Downing Street source made clear that Mr Cameron had "four boats firmly in mind".

He added that the Prime Minister had not seen "any evidence that the job of deterrence can be done properly with less".

The Tories and the LibDems have delayed the so-called "main gate" decision on what a replacement will look like until after the next general election.

But both parties have already engaged in spats on the issue.

The review into potential alternatives to a like-for-like replacement was a concession to the LibDems during the negotiations which set up the Coalition with the Conservatives in 2010.

It is thought Labour believes any significant savings from cutting the number of boats would not be realised until almost 2030, because of the length of time to build each submarine.

Labour remains "open minded" about what form a Trident replecement should take.