Liberal Democrat members are pushing the coalition to carry out a full review of plans to replace Trident.
A motion to be debated at the party’s annual conference warns a like-for-like replacement for the nuclear deterrent could mean widespread military cuts.
It says the decision not to include Trident in the upcoming defence spending review is “untenable”.
The cost of a replacement for the system is expected to be between £20bn and £30bn.
The debate comes as the Lib Dem conference, the party’s first since it formed a coalition government with the Conservatives in May, comes to a close.
The Lib Dems oppose an identical replacement for Trident, arguing it was designed for the Cold War, but the Tories are strongly in favour of one.
In their coalition agreement, the parties said they would carry out a study aimed at exploring cheaper options.
In July, Chancellor George Osborne said the cost of replacing Trident would have to come from the Ministry of Defence’s budget, rather than directly from the Treasury.
The emergency motion brought before the Lib Dem conference, taking place in Liverpool, calls for an extension of the defence spending review to cover Trident.
It suggests looking at “cost-saving options” such as ending continuous at-sea patrols and lengthening the operational lives of the Vanguard submarines used to carry nuclear missiles.
The motion also says the review should make it clear how much Trident replacement would cost in terms of cuts to troop numbers and equipment programmes.
Trident consists of four submarines that can deploy ballistic missiles carrying nuclear warheads.
Labour, which also backs a like-for-like replacement, argues that delaying a decision could risk Britain’s continuous “sea deterrence” and cost industry billions.
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