The UK could cut back its Trident submarine force and save billions of pounds "without sacrificing" its nuclear deterrent, a report suggests.
The paper, by military think tank the Royal United Services Institute, said Britain did not always need to have at least one nuclear submarine at sea to be sure of deterring an attack.
The UK currently operates a continuous at-sea nuclear weapons system.
Defence Secretary Phillip Hammond said he did not back a part-time deterrent.
The UK has had at least one submarine on patrol at any given time for more than 40 years and has used the Trident system since the 1990s.
The Conservative Party is currently committed to a like-for-like replacement for the existing four-boat fleet needed to maintain round-the-clock patrols - at an estimated cost of £20bn - if they win the next general election.
Labour also supports a continuous at-sea deterrent (CASD) system.
But the Liberal Democrats are in favour of reducing the number of Vanguard submarines from four to three, arguing the existing system was designed for the Cold War era.
A government review published in July - which had been requested by the Lib Dems - set out possible options for replacing the UK's Trident nuclear system, and a final decision on the issue is to be made in 2016, after the next election.
Defence Secretary Philip Hammond has criticised the Lib Dem proposals as "naive", warning the party against taking a "huge gamble" with national security.
He said: "Our submarine-based continuous-at-sea deterrent, provided by the Royal Navy for almost 50 years, is the ultimate safeguard of our national security.
"No alternative would be as effective at deterring threats now or in the future and that is why we are progressing with our commitment to a like-for-like replacement. We will not put the nation's security at risk by downgrading to a part-time deterrent."
However, the Rusi paper - by analyst Hugh Chalmers - said that even if there was not always a submarine on patrol, Trident would still represent a powerful deterrent to aggressors.
It said the mere existence of submarines - whether active or inactive - would have the desired effect of dissuading a potentially hostile state from threatening or blackmailing the UK, if those potential enemies believed the UK's nuclear forces could be deployed "in a crisis".
The Rusi paper acknowledged that abandoning CASD could create "unnecessary risks" at a time of "strained" defence budgets, but it maintained the system should be subject to a cost-benefit analysis.
It also went on to say that, while the UK remained a member of Nato, it would retain the ability to flex considerable nuclear strength.
"In this case, if the UK can implement a flexible structure in which to operate a non-continuous nuclear force without sacrificing any financial or political gains it hopes to make, a step away from CASD could be one of several legitimate pathways in the UK's nuclear future," it added.
Liberal Democrat Sir Nick Harvey MP said: "I very much welcome Rusi's report today. It's pleasing to see this well-respected and expert organisation coming to this issue with an open mind.
"The report concludes that a step away from CASD is a legitimate option, which is quite right: the Lib Dems have long argued that the nuclear status quo is unnecessary and unaffordable."
He added: "We need to come down the nuclear ladder in a way that is consistent with the world today. We no longer need to be able to fire on Moscow at a moment's notice. We cannot escape uncertainty, but we have finite defence resources and a Cold War nuclear capability has little relevance to the range of security challenges we're likely to face in the future."