Peace activists cursed "an historic opportunity missed" yesterday after Liberal Democrats backed their leaders' "Trident-lite" nuclear weapons programme.

The ageing nuclear submarines are due for a £65 billion overhaul by 2016, which anti-war and anti-austerity movements warn could top £120bn over the fleet's lifetime.

But the Lib Dem leadership have insisted that Britain should keep the fleet, but drop continuous at-sea patrols and cut the number of subs from four to two.

Vice-chairwoman on federal policy Julie Smith told delegates that ministers had to remain "credible" to continue bargaining with other nuclear-armed nations and their Tory coalition partners.

But Calderdale delegate Nick Taylor ridiculed the move.

"We've kept the submarines, but we've taken the bombs off them and we'll only deploy them in a crisis," he said.

"God almighty help us."

Glasgow South delegate Paul Coleshill compared the nuclear stockpile to an antique sports car that he had no use for.

"My garaged car costs a fortune to maintain, store and run.

"I've been offered a replacement - if I only use it on Mondays, Wednesdays and Sundays it could save me 6 per cent," he said to laughter.

But Treasury Chief Secretary Danny Alexander insisted he had not been ordered to draft the policy as a cost-cutting exercise.

He said upgrading the fleet would represent "one of the most significant acts of disarmament that this country has ever undertaken."

His review has been pilloried for lacking a no-nukes option, but Mr Alexander was adamant.

"The fact is whatever option you choose - whether it's unilateral disarmament or this one - it will not save money."

The motion to scrap Trident's replacement fell in silence, 228 to 322.

Campaign for Nuclear Disarmament's Kate Hudson said the vote was "clearly an historic opportunity missed."

The Lib Dems had a firm anti-nuclear record while in opposition, she said.

"But unfortunately some in the party seem to have seen the 'Trident-lite' option as a way to ease potential coalition negotiations in 2015."