* Britain's nuclear submarine system up for renewal in 2016
* Two ruling parties split over possibility of scaling down (Recasts, updates with report, Alexander, Cameron)
By Andrew Osborn
LONDON, July 16 A rift in Britain's coalition government over nuclear weapons widened on Tuesday as the two ruling parties set out starkly different visions of how they should be deployed in future.
The ruling Conservative party called its junior coalition partner, the Liberal Democrats, "naive or reckless" for suggesting that Britain's multi-billion pound submarine-based Trident nuclear missile system could be slimmed down.
With a parliamentary election in 2015 and a decision due soon after that on how to replace the ageing system, the two parties drew widely differing conclusions from a government review commissioned to see if there were any credible alternatives to the current nuclear deterrent.
The review concluded that the best option was to replace the current system, which uses Vanguard-class submarines armed with Trident missiles - on a like-for-like basis, but raised the possibility of ending Britain's continuous sea-borne deterrent and of operating fewer than the current four submarines.
Prime Minister David Cameron, a Conservative, suggested the report meant no policy change was necessary.
"Government policy remains as set out," he said in a statement. "We will maintain a continuous deterrent and are proceeding with the programme to build a new fleet of ballistic missiles submarines."
But Danny Alexander, the Liberal Democrat minister who oversaw the review, said he thought it showed there was scope to scale back the nuclear deterrent, stop the practice of having one submarine at sea 365 days a year, and reduce the number of nuclear submarines from four to three or even less.
"We have a big decision to make in 2016, and this study shows that there are credible alternatives that don't compromise our security but do allow us to move on from the Cold War," he told the Royal United Services Institute think-tank in London.
"We can adapt our nuclear deterrence to the threats in the 21st century by ending 24 hour patrols when we don't need them, and buying fewer submarines."
Scaling down to three submarines would save 4 billion pounds ($6.04 billion) over the life of the system, he added. Deploying two boats would be even cheaper, a potentially significant factor at a time when Britain is mired in debts.
Conservative Defence Minister Philip Hammond said Alexander's proposals were a risky false economy and promised "a tiny saving for a huge gamble with Britain's security."
"I think it's frankly either naive or reckless," Hammond told BBC radio.
"We do not believe that, with nuclear threats, if anything, proliferating, with more countries seeking to get nuclear weapons, this is the time to downgrade. We've got countries like Iran and North Korea attempting to build nuclear weapons."
Nobody knew who the potential adversary of the future would be, he added.
The opposition Labour party, which is ahead in most opinion polls, said it saw nothing in the report to change its own commitment to a continuous sea-borne deterrent.
The policies of Britain's three main political parties on the issue could influence the make-up of any future coalition.
Trident missiles are built by Lockheed Martin Space Systems and are also used by the U.S. navy. The submarines are operated from Scotland, which is holding an independence referendum next year. Scots nationalists oppose having nuclear weapons on their territory. ($1 = 0.6621 British pounds) (Additional reporting by William James and Sarah Young; Editing by Robin Pomeroy)