Aldermaston is responsible making Britain's Trident nuclear warheads
The government has sold its last remaining shares in the Aldermaston Atomic Weapons Establishment in Berkshire to an American company.
The move means Britain no longer has any stake in the production of its Trident nuclear warheads.
Opposition MPs have criticised the sale, but the Ministry of Defence said Britain's "sovereign interests" had been protected.
The fee paid by California-based Jacobs Engineering has not been disclosed.
The sale of British Nuclear Fuels' stake means Jacobs has control of one third of Aldermaston's operating company, AWE Management.
The other two thirds were already in private hands. They are split equally between American defence giant Lockheed Martin and the British plc Serco.
Aldermaston is responsible for the production of warheads for the Trident nuclear deterrent programme and its planned replacement.
The Conservatives said the government must explain the move.
Defence spokesman Gerald Howarth said Aldermaston was "critical to Britain's nuclear deterrent capability".
Opposition MPs on the sale of the Aldermaston weapons plant
"I'm not anti the United States," he told the BBC. "What I'm absolutely determined is that the United Kingdom should have total control of its independent nuclear capability.
"And I think the people of this country will want to know that... their politicians are in command of the research and development of this capability.
"We're happy to have the support of the United States, but policy-wise it must be in the hands of the United Kingdom."
Liberal Democrat defence spokesman Nick Harvey said: "The whole argument used for Britain having a separate weapons establishment is that this is required by the non-proliferation treaty, as technology sharing is not allowed.
"We must therefore query the rationale of a US company having a majority shareholding in AWE. How does this all square?"
The sale was announced in a one paragraph statement on the BNFL website.
The Lib Dems said it was staggering that Parliament had not been properly informed.
The safe operation of AWE will remain unaffected by the sale
Ministry of Defence spokesman
Their Treasury spokesman, Vince Cable, also said it was "a terrible time" for the government to sell its share because it would not get the best possible return for it.
The Ministry of Defence insisted Britain's strategic interests had been "taken into account".
A spokesman said: "It is the UK government, not AWE, that sets the UK's nuclear policy.
"UK sovereign interests remain protected at all times, as does the independence of the UK deterrent. The safe operation of AWE will remain unaffected by the sale."
The government said it had retained a "special share" in AWE, which would enable it to fire the operators or intervene on site if necessary.
Aldermaston has been the headquarters for the UK's atomic warhead capability for more than 50 years.
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