"The decision will be taken later this year," he added, refusing however to specify whether it would be put to a vote following a promised "fullest possible debate" in the House of Commons.

Previously, the government has said that a decision to update or replace submarine-launched Trident missiles would be made before the end of the current parliament in 2010.

Last Thursday Blair's finance minister and heir apparent, Chancellor of the Exchequer Gordon Brown, told a business audience in London that he was in favour of keeping Britain's nuclear deterrent.

It was put in place in the 1980s by then prime minister Margaret Thatcher, when the Soviet Union -- and not elusive global terrorist groups such as Al-Qaeda -- was seen as the primary security threat.

It is based on four Royal Navy submarines fitted with US-built Trident missiles which are due to become obsolete in the 2020s. One of the submarines is always on patrol but the missiles are no longer pre-targeted.

Replacing the deterrent is likely to cost anywhere from 10 billion to 25 billion pounds (14.6 billion to 36.4 billion euros, 18.6 billion to 46.1 billion dollars), observers say.

Critics say the end of Trident's useful life should be an opportunity for Britain to make good on the disarmament committments by the world's nuclear powers in the Non-Proliferation Treaty.

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