The government is being urged to cancel the "nonsensical" replacement of the Trident nuclear submarines and use the money to "revitalise" the armed forces.
Liberal policy think tank Centreforum says Britain is "sleepwalking" into replacing Trident at a cost of £25bn, while cutting the navy, Army and RAF.
It argues there is no current or medium-term threat to justify the cost.
The MoD said the government was committed to maintaining a continuous submarine-based nuclear deterrent.
In May 2011, the then Defence Secretary Liam Fox approved the £3bn first design stage for the new submarines.
However, the coalition government has delayed a final decision until after the next election amid concerns from the Liberal Democrats about the cost.
Shadow defence secretary Jim Murphy supports the renewal of Trident, although many backbench Labour MPs are opposed to the idea.
Centreforum proposes that Britain should retain the capability to design and build nuclear weapons in case they are needed in the future.
But, it says, the UK's ballistic missile submarines should now be converted to an attack fleet and equipped with conventionally armed cruise missiles.
It says the government should invest the billions saved by cancelling Trident in Britain's conventional forces, which would otherwise face further cuts by 2020.
Report author Toby Fenwick said: "Replacing Trident is nonsensical. There is no current or medium-term threat to the UK which justifies the huge costs involved.
"A critical assessment of the UK's strategic position and military requirements leads to a clear conclusion: Trident makes no effective contribution to our security. Cancelling it will provide a unique opportunity to rebalance and revitalise Britain's forces for the 21st century."
The issue of a like-for-like replacement of Trident is one that has divided the coalition.
The Ministry of Defence said that as part of the coalition agreement, the Liberal Democrats would continue to explore the possibility of an alternative approach to delivering a credible nuclear deterrent, with a report to the prime minister and deputy prime minister due to be published later this year.
Liberal Democrat MP Sir Menzies Campbell, who is chairing the cross-party commission looking into the nuclear issue, said the report was ill-timed.
MPs were still considering a variety of evidence from experts and confidential sources and it seemed "premature to jump to a conclusion", he told BBC Radio 4's Today programme.
The chief executive of Centreforum, Chris Nicholson, said: "We are pleased that the government is carrying out a study of alternatives to Trident.
"It is important that the option of removing Trident from service immediately is considered as part of this review."