The coalition parties are at odds after a Lib Dem-prompted government report set out options for replacing the UK's Trident nuclear weapons system.
The Lib Dems favour reducing the number of Vanguard submarines from four now to three, saying the existing system was designed for the Cold War era.
Defence Secretary Philip Hammond said it would be "naive or reckless" not to have a like-for-like replacement.
But Lib Dem Danny Alexander accused him of trying to "rubbish the report".
A final decision on the issue is to be made in 2016, after the next election.
The UK has had a continuous-at-sea nuclear weapons system, with at least one submarine on patrol at any given time, for more than 40 years and has used the Trident system since the early 1990s.
While the government remains committed to Trident, the coalition partners agreed to undertake a review amid disagreements over future capability and cost.
The review makes no recommendations but set outs a range of options.
Its main findings are:
- There are alternatives to the current posture which would enable the UK to inflict "significant damage" and deter aggressors
- Submarines could potentially be operated at "reduced readiness" when threat levels are lower
- A continuous-at-sea presence is the most "resilient" posture and guarantees the quickest response
- Land and air-based delivery systems effectively ruled out
- An entirely new system, using cruise rather than ballistic missiles, would be more expensive than renewing Trident
The UK's current four-submarine fleet will reach the end of its lifespan in the 2020s and one of the main arguments surrounds how many "successor" submarines - which take 17 years to build - should be commissioned.
The report suggests four boats would be required to maintain a continuous-at-sea presence and a smaller fleet would risk "multiple unplanned breaks" in 24-hour patrolling and could affect the UK's ability to respond in crises.
It says the UK could still operate a nuclear weapons system with three or even two boats but that would depend on "political confidence" that there was no chance of an unexpected pre-emptive attack and more regular patrols could be reconstituted.
But Mr Hammond told the BBC that nuclear submarines were the "most complex man-made object on earth" and reducing the numbers available would leave the UK extremely "vulnerable".
"Just because we do not perceive an immediate threat today, does not mean there would not be a threat over the 60-year odd time horizon we are looking at," he said.
"The truth is, at the end of the day, we can have continuous-at-sea nuclear deterrents or we can have a part-time deterrent. The part-time deterrent will save us only trivial sums of money."
But Chief Secretary to the Treasury Danny Alexander accused Mr Hammond of trying to "rubbish the report" and said his Conservatives colleagues "were worried about losing the argument".
The UK's nuclear policy had "not moved on very much since the end of the Cold War", he said, and the review showed there were "credible alternatives" to the current arrangements.
"We can move on by ending 24-hour patrols when we don't need them and buying fewer submarines," he said.
"That way we can move down the ladder of disarmament as a country without compromising our national security."
Critics have questioned whether the UK can continue to pay for Trident in its current form. The government estimates renewal costs will be between £15bn and £20bn but anti-nuclear campaigners say the figure will be much higher.
Mr Alexander said £4bn would be saved in the medium to long term from moving to three submarines but the Ministry of Defence says the current cost of operating the Trident fleet is about 5% of the annual £34bn defence budget.
Parliament will debate the findings of the report on Wednesday.
Labour said it remained committed to maintaining the "minimum credible independent nuclear deterrent" and it believed that was best delivered through a continuous-at-sea submarine presence.
"It would require a substantial body of evidence for us to change that but this review does not appear to offer such evidence," shadow defence secretary Jim Murphy said.
The SNP, which has vowed to remove nuclear weapons from Scottish soil if it wins an independence referendum next year, said the review was "not worth the paper it is written on".
"The Westminster establishment seem to have forgotten that Trident is based in Scotland, and neither the people nor parliament of Scotland want it here," said its Westminster leader Angus Robertson.
"This review is in denial, and panders to the vanity of the Westminster system which wants to keep this out-dated, dangerous arsenal of nuclear weapons on the Clyde."