By Peter Apps, Political Risk correspondent
FASLANE, Scotland (Reuters) - Outside Her Majesty's Naval Base Clyde, campaigners who have fought for decades for Britain to abandon nuclear weapons believe that they are closer than ever to victory.
In the 1980s, then Prime Minister Margaret Thatcher brushed aside the protesters, saying that in the Cold War, atomic arms made the world a safer place.
Now, groups such as the Campaign for Nuclear Disarmament believe their time has come. The referendum on Scottish independence and the looming retirement of the current Trident system herald change, they say.
The handful of activists who stay overnight in a camp of brightly-painted caravans and peace signs believe that Faslane's ballistic missile submarines will be the last of their kind.
"I think we do have a chance now," said activist Angus Chalmers. "Everything is coming together. It's possible to make the case we don't need these weapons in a way that just wasn't possible during the Cold War."
The Scottish Nationalist Party government that runs Scotland's devolved government say the nuclear weapons must go if they win the September 2014 referendum on independence.
Prime Minister David Cameron visited Faslane this month to argue a vote for independence could jeopardize British and Scottish security. The confrontation with North Korea showed that deterrent was as vital as ever, he said.
But that will not be the end of the battle. The British government has already extended the life of the current four Trident submarines towards 2030 and is spending 1.5 billion pounds already on preliminary design work for their successors.
A final decision will be made after the 2015 election, with the Defence Ministry estimating the cost of replacement at around 20 billion-25 billion pounds.
Opponents say money could be better spent elsewhere - it is enough to hire 120,000 new nurses a year for a decade.
Public opinion may be shifting. A 2009 Guardian/ICM poll showed for the first time most of those surveyed favoring outright nuclear disarmament over replacement.
"If the next British government decides that Britain is to remain a nuclear weapons state, it must be prepared to face - and defeat - the most articulate and agile single issue opposition it has ever faced," ballistic missile submarine captain Cmdr Andy Corbett told a panel at London's Royal United Services Institute last year. "It needs to be preparing now."
The United States, current and former officials said, is extremely keen Britain remains a nuclear power.
They have the closest co-operation of any two nuclear-armed states, with considerable shared planning and research and with London effectively leasing the U.S.-built Trident missiles.
DEPENDING ON SCOTLAND
Those in the immediate area are worried. Thousands were thrown out of work when the U.S. Navy abandoned its Scottish ballistic missile submarine base at nearby Holy Loch.
The Defence Ministry says it is not carrying out any contingency planning for Scottish independence as ministers believe the union will continue.
Ironically, it was the unpopularity of Thatcher's government in Scotland - as well as Tony Blair's Labour administration - that opened the door to the SNP and next year's referendum.
For now, opinion polls suggest the pro-independence camp will lose - although with up to a fifth of the electorate undecided, those on both sides say things could change.
What is clear is that in its current form - four submarines, one always at sea carrying eight missiles and 40 warheads - the British deterrent relies almost completely on its Scottish base.
While the warheads themselves are assembled in England at the Atomic Weapons Establishment in Aldermasten, Faslane is the only facility capable of handling the nuclear-armed submarines.
The two most promising alternate locations, experts say, would be Milford Haven in Wales or the existing naval dockyard at Devonport, Plymouth. But both come with problems.
If Scotland were to vote to leave the United Kingdom, moving the deterrent to Wales, which also has a vocal nationalist movement, might seem risky.
Devonport, meanwhile, stands at the centre of a densely populated area that would make any nuclear accident potentially harmful to many more people.
"Everyone in London is just hoping the referendum doesn't see a victory for the "yes" vote," says Mark Fitzpatrick, a former senior U.S. official specializing in nuclear issues and now a senior analyst at London's International Institute for Strategic Studies.
"It would be hugely difficult for what was left of the rump United Kingdom if Scotland left."
Anxious to rebuff talk of many job losses, the SNP says Faslane would remain a military centre of an independent Scotland, home to a navy of patrol boats and a handful of larger craft.
Cutting a deal on the nuclear deterrent would almost certainly be a part of negotiations between London and Edinburgh on dividing military resources. Some suggest Faslane could remain a "sovereign base" still under London's control.
"The issue of Trident is a totemic one in Scottish politics," said Angus Robertson, SNP leader in the Westminster parliament and its spokesman on defence issues. "We would want it gone."
The Defence Ministry says that if Britain is to retain a permanent nuclear deterrent, Trident replacement offers by far the best value for money.
Other potential solutions - fewer submarines, using low flying cruise instead of ballistic missiles - would be much less resilient and could be destroyed much more easily.
The ruling Conservative party says it is committed to replacing Trident with a similar submarine-based ballistic missile system. Junior coalition partners the Liberal Democrats, however, have long been opposed and are conducting their own study into potential alternatives.
The opposition Labour Party, which most opinion polls suggest will win or at least emerge as the largest single party after the next election, remains divided. The party's support for unilateral disarmament at the height of the Cold War is seen contributing hugely to its electoral defeats and it is seen likely to back a submarine missile system.
"Prime ministers do not want to take the decision to be the person to give up our deterrent," said Sir Richard Mottram, former permanent secretary in the British civil service.
"They worry that in 2050, something horrible will have happened and they will be the person who goes down in history as responsible."
(Reporting By Peter Apps; Editing by Angus MacSwan)