LONDON (Reuters) - Prime Minister David Cameron reopened a rift in his coalition government over the future of Britain's nuclear deterrent on Thursday, saying potential threats from countries such as Iran and North Korea meant it could not be scaled back.
Cameron's comments put him at odds with his Liberal Democrat coalition partners who want to find a cheaper alternative to Britain's multi-billion pound submarine-based Trident nuclear missile system to try to save money at a time when the nation's finances are mired in debt.
Cameron, the leader of the Conservative Party, the senior member of the two-party coalition, said he had not seen any evidence there were cheaper ways of providing a credible alternative to the "ultimate weapon of defence", saying the nuclear threat had grown since the end of the Cold War.
"Iran continues to defy the will of the international community in its attempts to develop its nuclear capabilities, while the highly unpredictable and aggressive regime in North Korea recently conducted its third nuclear test," Cameron wrote in The Daily Telegraph newspaper.
"We need our nuclear deterrent as much today as we did when a previous British Government embarked on it over six decades ago."
Cameron's intervention coincided with a visit he made to Scotland, where the submarines are based, to welcome the crew of one vessel back from a patrol.
He used the trip to explain why he thought Scots should vote to stay part of Britain in an independence referendum next year, but the pro-independence Scottish National Party (SNP) accused him of scaremongering after he said Scottish defence jobs were more secure while Scotland was part of Britain.
The SNP has said it does not want nuclear weapons in an independent Scotland.
A Trident renewal decision will not be taken until after the next 2015 general election, but the fact that Britain's four Vanguard class nuclear submarines reach the end of their service lives in the 2020s means it cannot be put off for long.
Britain's big debts will weigh on any decision to commit billions of pounds to a successor system to Trident.
"We don't see why, at a time of deep austerity, we should be spending billions and billions of pounds on a nuclear missile system which can flatten Moscow at the touch of a button," a senior Lib Dem source told Reuters.
"We're talking about eye-watering amounts of money at a time when there isn't much money about. All three political parties will have to work hard to make the case for a replacement."
Cameron played down the costs, saying Britain's nuclear capability cost less than 1.5 percent of what the government spent on welfare benefits each year.
The opposition Labour party, which is ten points ahead of Cameron's Conservatives in the polls, is reviewing its own policy on Trident. For now, one of the four submarines is always on patrol. One money-saving idea is to pare that schedule back.
Trident missiles are built by Lockheed Martin Space Systems and are also used by the U.S. navy.
Editing by Mike Collett-White