Britain's new generation of nuclear warheads are being developed at a Berkshire site ahead of any government decision, a campaign group alleges.
AWE is the headquarters of Britain's nuclear development programme
Aldermaston Women's Peace Campaign (AWPC) wrote to Downing Street stating nuclear warheads are being developed at the Atomic Weapons Establishment (AWE).
In October, building work at the Aldermaston site led to claims from Greenpeace of new nuclear developments.
The MoD denied new weapons were being developed and insisted AWPC was wrong.
Prime Minister Tony Blair announced plans late last year to upgrade Trident at a cost of up to £20bn with the commons to vote on future plans for any nuclear deterrents in March.
In a letter to the Commons Defence Select Committee, the AWPC stated: "Plans for warhead replacement are already well advanced - we suggest that the decision has already been made.
"The Defence White Paper [December's paper on the Trident system] fails to reveal the true extent of the government's progress on developing a successor system."
Labour MP Kevan Jones dismissed the AWPC's ideas as a "conspiracy theory".
A spokesman for the MoD said: "They've got it wrong. Work on the warheads will not be needed yet - we've already announced that.
"We are reducing the number of warheads, because the threat has changed. There's not much to be gained by developing new warheads.
"And the decision whether or not we need or don't need to replace warheads will not be taken until the next parliament."
He added that additional investment at the AWE research site was "sustaining facilities and skills to maintain existing stockpiles".
"Before if you had a stockpile of 100 every few years we'd dip in and test 10 - now we can't do that, so how do you ensure all work?
"Modelling and laser technology - that's what a lot of the investment is about," he added.
The building work, at a cost of £1bn, and the creation of new jobs had led to claims from Greenpeace that the Article 6 of International Nuclear Non-proliferation Treaty, relating to nuclear disarmament, was being contravened at the site.