Britain's nuclear weapons capability should reduced by half, retaining only 100 warheads, the Lib Dems say.
Sir Menzies Campbell said a decision on the Trident system should be made after 2014 when a clearer picture of nuclear threats and proliferation is known.
But by cutting the number of warheads now, Britain would show that "nuclear disarmament is back on the international agenda," he said.
A white paper outlining options for replacing Trident is due on Monday.
Sir Menzies said the party was "in favour of the most cost effective replacement for the current Trident system being a submarine system based on the Trident missile of three boats carrying no more than 24 warheads each".
He said the party believed that any deterrent should be at a minimum level, and 100 warheads was enough to ensure Britain's security.
TRIDENT MISSILE SYSTEM
"There is a measurable danger that if North Korea and Iran are confirmed over the next decade as nuclear states, they will set in train a course of nuclear proliferation which will materially alter the strategic situation.
"It would be unwise at this time for Britain to abandon its nuclear weapons altogether.
"But a deterrent of approximately half the current size, and extending the life of the current submarine system, would be sufficient to provide for Britain's ultimate security until we have more certainty about proliferation," Sir Menzies said.
The Lib Dems will debate the nuclear issue at their spring conference.
Prime Minister Tony Blair and Chancellor Gordon Brown have both indicated they want to see the UK keeping its own nuclear weapons system.
Although there have been reports of some disagreements in Cabinet, ministers are to outline their favoured option - expected to be a replacement for the Trident system - in the white paper.
There will then follow a three-month consultation on the plans and a vote by MPs in the House of Commons.
Anti-nuclear campaigners say they fear the government has already decided to go ahead with replacing Trident.
Critics say the cost of replacing Trident - estimated at up to £25bn - would be better spent elsewhere, particularly as nuclear weapons would be useless in the fight against international terrorism.
Ministers want a quick decision to ensure any replacement is ready when Trident's working life ends in 2024.
Britain has 16 Trident missiles based on each of the four nuclear submarines.
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