LONDON (Reuters) - The United States has quietly withdrawn its last nuclear weapons from Britain after more than half a century, a watchdog said on Thursday.

Anti-nuclear campaigners welcomed the apparent end of an era, brought about by changes in warfare and world politics rather than their dogged protests over the decades.

The Federation of American Scientists, which studies the U.S. nuclear arsenal, said in a report that Washington had removed its last atomic bombs from the Royal Air Force base at Lakenheath, where they had been stationed since 1954.

An anti-war protester holds up a plastic rocket that reads Lakenheath, referring to England's largest US Air Force Base, during a protest in London November 19, 2003.

The withdrawal has not been announced officially, but was confirmed by several sources, the report's author, nuclear weapons expert Hans Kristensen, wrote.

The Ministry of Defence declined to comment. A U.S. air force spokeswoman at the base said Washington's policy was not to comment on the location of nuclear weapons.

If true, the withdrawal means U.S. nuclear bombs in Europe are kept at just six bases -- mainly at Turkey's Incirlik air base and Aviano in Italy, but also in Belgium, Germany and the Netherlands, Kristensen wrote.

Bombs dropped from planes stationed at air bases play a much smaller role in the U.S. nuclear arsenal than they once did, having mostly been replaced by warheads attached to missiles.

But U.S. nuclear weapons have long made Lakenheath in southeast England a magnet for protests, which peaked in the 1980s when many Europeans feared obliteration in a nuclear war between the West and the Soviet Union.

"The news that these bombs have been withdrawn from Lakenheath is extremely welcome," said Kate Hudson, head of the Campaign for Nuclear Disarmament, or CND.

"We would like official confirmation from the government that this has happened and believe an open admission will be a confidence-boosting measure for future disarmament."

At the height of the Cold War, the United States had more than 7,000 nuclear weapons in Europe. Most were withdrawn in the early 1990s, and today, Kristensen estimates the number at fewer than 240.

But the CND's Hudson said their final removal would not effect the campaign against deploying U.S. missile defence systems in Britain -- which still has its own nuclear arms.

Kristensen he said it was "a puzzle" that the withdrawal had not been announced at a time when the West is arguing with Russia over weapons cuts.

"By keeping the withdrawals secret, NATO and the United States have missed huge opportunities to engage Russia directly and positively about reductions to their non-strategic nuclear weapons, and to improve their own nuclear image in the world in general," he wrote.

The report can be found on the Internet at www.fas.org/blog/ssp/

Twenty years after the Cold War, should the United States still station nuclear weapons in Europe? Check out the Reuters Global News Blog at blogs.reuters.com/global/

(edited by Richard Meares)

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