The UK's newly-privatised nuclear bomb store on the Clyde has managed to avoid independent regulation, prompting safety fears.

The Sunday Herald has learned that the Royal Naval Armaments Depot at Coulport narrowly escaped being licensed for inspection by the UK's nuclear safety watchdog. This is despite its operation being taken over in January by a group of private companies headed by the US arms dealer, Lockheed Martin.

The news prompted one former senior Ministry of Defence (MoD) official to warn that the move is putting workers and the public at risk. It has also angered politicians and trades unions, who suspect a "cosy stitch-up" to save money.

MoD nuclear sites have historically had Crown immunity from prosecution as they have not been covered by nuclear safety legislation. However, if their management was put into private hands, this changed.

In 1997, the bomb-making sites at Aldermaston and Burghfield in Berkshire were licensed under the Nuclear Installations Act and subject to independent inspections after being handed over to private firms.

But this did not happen at Coulport. Documents released under Freedom of Information laws show the firms now running the weapons store escaped safety licensing because the MoD's continuing oversight of the site was deemed "the absolute minimum that is acceptable".

After a visit in November, the UK Government's Office for Nuclear Regulation (ONR) decided Coulport did not need to become a nuclear licensed site. This followed arguments from site staff that new management arrangements were sufficient to "guard against potential licensing by ONR".

The ONR has, however, said it will keep the decision under review.

Fred Dawson, who retired as head of the MoD's radiation protection policy team in 2009, accused the MoD of having its cake and eating it, by contracting out its operations while "still clinging to Crown exemption".

He noted that the nearby nuclear submarine base at Faslane and the Vulcan naval reactor testing facility at Dounreay in Caithness were also exempt from civil safety regulation.

"The MoD fears it cannot meet the same safety standards as the civil sector," he said. "This also by implication means workers on MoD sites such as Faslane, Coulport and Vulcan – and the public – are at greater risk than at civil nuclear sites."

Coulport, by Loch Long, houses Trident missiles and their nuclear warheads, and is managed by the ABL Alliance, made up of AWE, Babcock and Lockheed Martin.

The SNP's defence spokesman at Westminster, Angus Robertson MP, said: "The decision to keep Coulport outside the civil nuclear safety regime bears all the hallmarks of a cosy stitch-up between government departments."

Ian Fraser, of the Public and Commercial Services union, called for the House of Commons Defence Select Committee to investigate.

The Coulport documents were obtained by the Nuclear Information Service, which called for the site to be under ONR legislation.

However, an ONR spokeswoman said that as long as Coulport remained part of the HM Naval Base Clyde, it did not have to be licensed.

The MoD argued that Coulport was different from Aldermaston because it was still "ultimately controlled" by the MoD and subject to regulation by the Defence Nuclear Safety Regulator, the ONR and other bodies. Their reports showed it had an "excellent safety record", said a spokesman.