* Defence Committee says independence threatens nuclear deterrent
* Lawmaker report urges Britain to draw up contingency plans
* Relocating nuclear submarine base would be costly and complex
By William James
LONDON, Sept 27 (Reuters) - The British government must urgently draw up a contingency plan for where it can put its nuclear arsenal in case Scotland, where the submarine-based nuclear missile system is now located, votes for independence, a parliamentary study said on Friday.
Scottish separatists have consistently lagged in opinion polls which show Scots have decided against independence by a ratio of 3 to 2. But a large number of voters - up to 28 percent by one poll - are undecided.
With less than a year to go to the Sept. 18, 2014, referendum, the issue of what to do with Britain's nuclear deterrent is urgent, lawmakers said.
"The possibility of Scottish independence represents a serious threat to the future operational viability of the UK's nuclear deterrent," said one of the conclusions of the year-long Defence Committee investigation into the impact of independence.
"The UK Government must now give urgent consideration to contingency options in the event of a 'Yes' vote," it said.
Britain is one of only five officially recognised nuclear-armed countries. Four submarines equipped to carry nuclear missiles - the Vanguard, Victorious, Vigilant and Vengeance - are based in deep-water lochs along the west coast of Scotland, and concrete missile storage bunkers are built into the Scottish hillside. At least one of the four is always at sea.
The Liberal Democrats, junior partner in the Conservative-led coalition government, want to reduce the number of submarines when the current class is decommissioned, but Prime Minister David Cameron has called any such plan "naive or reckless".
If Scotland votes to break away, Britain would face a huge, expensive and time-consuming task to relocate the system.
The Ministry of Defence confirmed on Thursday it currently has no contingency plan.
"No contingency plans are being made to move Trident out of Scotland," a Ministry of Defence spokesman told Reuters.
"The scale and cost of any potential relocation away from Faslane would be enormous. We are confident that the Scottish people will vote to remain a part of the United Kingdom."
Defence Minister Philip Hammond said during an evidence session for the report that it was "unhelpful" to speculate over any possible relocation sites, but that ultimately he was confident it could be done.
"I am confident that we would be able to solve this problem, but it would cost a significant amount of money," Hammond told the committee in July.
A Trident renewal decision will not be taken until after the next 2015 general election. Britain's four Vanguard class nuclear submarines reach the end of their service lives in the 2020s.
Keith Brown, a minister in the devolved Scottish government, called the current nuclear programme a "huge waste of money" and said an independent Scotland would focus its military efforts differently.
"An independent Scotland can halt the disproportionate cuts to our defence footprint and deliver the defence jobs, the equipment and the security that Scotland needs," he said.
Trident missiles are built by Lockheed Martin Space Systems and are also used by the U.S. Navy.