A major upgrade of a top-secret system for targeting Trident warheads has sparked fears that the UK's nuclear weapons are effectively under the control of the USA.

The Ministry of Defence (MoD) has completed an eight-year overhaul of the high-security computer software used to determine which cities and military facilities across the world would be hit by nuclear bombs in the event of war. Details of the upgrade are “classified”.

Critics claim that the UK system is critically dependent on US software, and that it’s the US president who is really in command. But this is denied by the MoD, which insists that that the targeting system is “one of the sovereign components” of the nuclear deterrent.

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An unpublicised article in an MoD technical journal in December 2015 disclosed that scientists and engineers had been thanked for bringing Trident’s targeting system into “a new era”. The upgrade project had required “zero tolerance of failure” and encountered “considerable challenges”, it said.

“This represents the culmination of eight years of design, development, production and test effort that comprises over 3.5 million lines of software code and in excess of 300,000 man-hours.”

Work was co-ordinated by the MoD’s strategic weapons project team based at the “UK Software Facility (UKSF)”. The MoD refused to say where this facility is located, but it is believed to be in a bunker hidden under a grass mound behind barbed wire near the town of Corsham in Wiltshire.

According to the MoD, the contractor for the upgrade was the hi-tech defence company, Mass. Quality assurance certificates available on the firm’s website give the address of UKSF as “Corsham Computer Centre”.

The MoD also declined to say what the purpose of the upgrade was, though experts have suggested that it could be to accommodate new missile technology designed to deliver “enhanced targeting capabilities”. It is likely to be part of a major US programme to extend the life of Trident missiles, which are carried by submarines based at Faslane on the Clyde.

Brendan O’Hara MP, the SNP defence spokesman at Westminster, accused the MoD of “arrogantly” pre-empting an expected parliamentary vote on whether or not to proceed with renewing the Trident nuclear weapons system. “We have always known the UK’s independent nuclear deterrent is not independent,” he said. “It’s American. It’s designed, manufactured, overhauled and even tested in the United States. The UK itself has admitted that it is inconceivable that the UK would ever use these weapons without the authority of the US.”

O’Hara added: “We know it will be an American commander in chief whose finger will be on the UK's nuclear button, not a UK Prime Minister. And in a few short months, the finger could be that of President Donald Trump.”

Dr Nick Ritchie, a nuclear weapons researcher at the University of York, argued that the UK was dependent on the US for vital targeting software. “The UK Trident system is to all intents and purposes an extension of the US fleet even if, technically, the Prime Minister can fire Trident at will,” he said.

Hans Kristensen, a leading nuclear expert from the Federation of American Scientists in Washington DC, thought it was impossible to know how independent UK nuclear weapons really were. “The UK system will have to be able to execute strikes independently, yet much of the system, architecture, and planning appears to rely heavily on the US,” he told the Sunday Herald.

The UK strategic weapons team at Corsham was authorised to receive US targeting information in support of NATO, he said. This required access on a daily and continuing basis to selected US nuclear planning information.

John Ainslie, coordinator of the Scottish Campaign for Nuclear Disarmament, pointed out that the new targeting system could send Trident missiles anywhere in the world. “In the past, Britain produced specific plans to launch nuclear weapons against Russia, today there is hardly a corner of the globe that is safe from Trident,” he said.

“This is such a specialised area that the UK is only able to do this with American help. At least one key component of the targeting system was purchased off-the-shelf from the US.”

An MoD graduate placement brochure quotes a maths student called only Judi who was working on nuclear weapons at the UK Software Facility. She says she travelled to the US twice because “a lot of the software the UK use operationally is provided by the US”.

However the MoD said that Judi had not been working on the targeting system upgrade. “The UK nuclear deterrent is completely operationally independent,” said an MoD spokeswoman.

“To that end the deterrent targeting systems are one of the sovereign components. Only the Prime Minister can authorise the firing of these weapons.”

Whether or not the system was based at Corsham and what improvements had been achieved by the upgrade were “classified”, she added. The upgrade formed part of other workstreams so its costs were “not readily identifiable.”