The UK Atomic Weapons Establishment, which manages the Aldermaston weapons site on behalf of the Ministry of Defence, is set to come under the control of US companies.

Fluor (NYSE: FLR - News) and Jacobs (NYSE: JEC - News) , two US engineering groups, have emerged as the only remaining bidders for the government's one-third stake in AWE, which it put up for sale last summer.

AWE is jointly owned by the government, through its stake in British Nuclear Fuels (BNFL), Serco, the British support services group, and Lockheed Martin (NYSE: LMT - News) , the US defence group. If either Fluor or Jacobs emerges as victorious, AWE will be two-thirds owned by US companies.

AWE is responsible for designing, manufacturing and decommissioning nuclear warheads for Trident, the fleet of nuclear submarines that is the UK's sole nuclear deterrent.

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Although the US already supplies the technology for the weapons, the prospect that the deterrent may end up majority-controlled by Americans is likely to prove controversial.

The recognition that the UK needed an independent nuclear deterrent after the second world war prompted the government to develop the facility. In 1946 Ernest Bevin, then foreign secretary, declared: "We have got to have this thing over here whatever it costs ... we have got to have the bloody Union Jack on top of it."

Under a 25-year contract awarded to the AWE consortium in 2000, if one of the three parties wants to sell, the other two have pre-emption rights to acquire the stake at market rate.

Both Serco and Lockheed have said they are keen to pick up the stake, but it is understood that the MoD would prefer it to go to a third party in order to inject more knowhow into the business. Latest estimates value the stake at about £100m ($195m, €133m).

Serco said: "We are in discussions with the MoD about how best to achieve the sale of the BNFL shareholding and how to continue to successfully deliver the AWE programme."

The sale of AWE is part of the government's plan to privatise all nuclear assets that were in public ownership. So far, foreign companies have been among the most successful in snapping up assets, sparking fears that Britain could lose valuable industry skills in the longer-term.

Toshiba, the Japanese company, bought Westinghouse, BNFL's nuclear reactor manufacturing arm, in 2006. America's Energy Solutions won the contract to run and decommission the UK's 10 Magnox reactors.

A government spokesman declined to comment on the identity of the shortlisted bidders but said: "Any bidder would need to satisfy MoD stringent security performance requirements."

BNFL said: "We can't comment on market speculation."

Investment bank NM Rothschild is handling the sale of the AWE stake on behalf of BNFL. A final decision on the disposal is expected in March.