Members of parliament are due to decide in March whether to back Prime Minister Tony Blair in supporting plans to modernise the ageing weapons system.

Lawmakers on the House of Commons Defence Select Committee were told Tuesday that building work at the Atomic Weapons Establishment (AWE) in Aldermaston, west of London, suggested the decision had already been taken.

In written evidence, the AWPC said "plans for warhead replacement are already well advanced".

"AWPC are calling on the government to come clean about the fact that at AWE Aldermaston, the Ministry of Defence has already started work on facilities to test, design and build new warheads, in advance of any parliamentary decision."

Committee member Kevan Jones, from Blair's governing Labour Party, said the claims were a "conspiracy theory".

An MoD spokesman told AFP the group had "got it wrong": warheads were not being replaced but work has been carried out recently to refurbish Aldermaston, whose facilities were last overhauled in the 1960s, he added.

Blair's proposals for Trident, including a new generation of nuclear submarines at a cost of up to 20 billion pounds (39.5 billion dollars), has been controversial.

Unilateral nuclear disarmament was Labour Party policy in the 1980s and many members are still opposed to retaining the nuclear deterrent.

Lobby group the Campaign for Nuclear Disarmament (CND) argue that to replace Trident would promote proliferation, particularly at a time when Blair and the West is opposed to North Korea and Iran having atomic weapons.

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