Review 'to find Trident alternatives too expensive'
Alternatives to Trident would either be impractical or more expensive, a review instigated by the Lib Dems will say, according to The Financial Times.
The Lib Dems and their Tory coalition partners disagree over whether to renew the submarine-based nuclear weapons system on a like-for-like basis.
The newspaper reports the review is to conclude that other delivery platforms would not work, or would cost more.
But it may suggest cutting submarine numbers could save billions of pounds.
The government said newspaper reports were speculative and the review would be published in due course.
A final decision on whether to build a new fleet of Vanguard submarines, which carry ballistic missiles equipped with nuclear warheads, has been pushed back until after the next election in 2015 amid disagreements within the coalition.
The existing submarines are due to be taken out of service in 2028.
The Lib Dems have long argued that the projected £20bn cost of building new submarines and ensuring that at least one is on patrol at any time - known as continuous at-sea deterrence - is too high and other options should be considered.
The Conservatives and military chiefs support Trident's renewal, believing it is the best guarantee of security for the UK.
When they entered government in 2010, the coalition partners agreed to a "value for money" review of Trident examining the cost of possible alternatives - overseen since last year by Lib Dem cabinet minister Danny Alexander.
According to the FT, the review - likely to be published before the end of July - is likely to reject alternative delivery mechanisms such as a land-based or airborne missile system.
The former would be deemed impractical, the paper said, as it would leave the UK unable to mount a meaningful response in the event of a nuclear attack while the latter would require a completely new aircraft which would be more costly.
The FT said the report would also cast doubt on the feasibility of using other submarines - such as the Astute Class due to come into service in the near future - to carry the nuclear missiles.
Conservatives have warned that this approach would mean an entirely new missile having to be designed.
But the newspaper said the report may float the idea of reducing the number of submarines from the current four to three or even cutting this back to two.
It quoted sources with knowledge of the report suggesting this could save at least £5bn in upfront design costs and £1bn in annual expenditure on crewing and maintenance per submarine.
Professor Malcolm Chalmers, research director at the Royal United Services Institute, said it was the "working assumption" in Whitehall that Trident would continue to operate off four submarines but the question of whether this could be reduced to three had always been "left open".
Any suggestion, he added, that the UK could maintain an effective deterrent with two submarines would be entirely new and would spark a debate going into the next election about whether a continuous at-sea presence was needed.
The review will not have any direct impact on the policy of the coalition government, which affirmed its commitment to Trident in its programme for government in 2010.
But it will inform debate on spending priorities going forward and on Lib Dem policy in the run-up to the next election.
A Lib Dem spokeswoman said the review had yet to be published but the party was "still looking at alternatives".
Prof Chalmers said the Lib Dems had never before spelled out in detail what the alternative should be and maintaining the current system, but with fewer submarines and at a lower cost, could be a potential way forward for them politically.
"If the leaks are correct, what it would suggest is that the Lib Dems would be getting closer to having an alternative of their own," he said. "They would have an actual policy rather than simply saying they do not agree with the current policy."
The Cabinet Office said it would not comment on speculation about what may or may not be in the report.
"No date has been set for the review but we still expect to report to the prime minister and the deputy prime minister in the first half of 2013," a spokesman said.
Labour, which backed renewal of Trident while in government, says it is committed to retaining an independent nuclear weapons system although it is prepared to look at options to do this in a way that reduces the burden to the taxpayer.