Liberal Democrat defence spokesman Sir Nick Harvey said he did not see any need for Trident to be patrolling the seas 24/7 while the UK has "no known nuclear adversary."
He told BBC Radio 4's Today programme the nuclear deterrent was "worth retaining", but said two submarines would provide a "perfectly meaningful deterrent", although not on a "continuous basis".
Retaining three submarines would enable the UK to have a "continuous at-sea deterrence", he added.
The main political parties are wrangling over whether the UK should spend billions renewing its nuclear deterrent.
If a difference of opinion on defence didn't get in the way of the current coalition - why would it be different for Labour and the SNP?