A Scottish peer has become only the second former defence secretary ever to argue the UK does not need its nuclear deterrent at sea around the clock.
Lord Browne of Ladyton warned the position – a cornerstone of the current Trident system based on the Clyde – could actually reduce Britain's safety.
The Liberal Democrats are arguing for a scaled-back deterrent.
But Labour says it backs a like-for-like replacement of the ageing submarines.
Trident is a major faultline within the Coalition, with the Tories also calling for a full replacement.
Supporters say having nuclear weapons at sea around the clock is crucial as a deterrent.
But critics argue it would be wasteful to spend an estimated £20 billion on a full-scale replacement.
Lord Browne said: "We need to shift the emphasis now to reduce the chances of any nuclear weapons being used anywhere."
But he said he does not support unilateral disarmament. It is thought Michael Portillo is the only other ex-defence secretary to have questioned the need for a round-the-clock nuclear deterrent at sea.
A former chief of the defence staff also reiterated his belief the UK Government should not go ahead with an unaffordable replacement for Trident.
Lord Bramall, 89, said a full-scale nuclear deterrent was not necessary.
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