The Ministry of Defence has been forced to reveal one of the most closely guarded secrets behind Britain’s nuclear weapons programme.

For years UK ministers have repeatedly refused to say where neutron generators – a vital component of the Trident warheads stationed on the Clyde – were manufactured. The information had to be kept secret for national security reasons, they said.

But now, having been confronted with undeniable evidence by the Sunday Herald, the MoD has admitted that the devices are imported from the United States. And in so doing, it has opened the ­Westminster ­Government to a barrage of criticism from assorted experts, politicians and campaigners, who now claim that Britain is not in full independent control of its own nuclear deterrent.

“This is another deceit of the British public by Westminster,” declared Dan Plesch, director of the Centre for International Studies and Diplomacy at the University of London.

“It’s another smoking gun in the ­arsenal of evidence that shows that there is no such thing as a British bomb. The weapon will become unusable should the Americans withdraw their support,” he said.

“We can’t use it if they don’t want us to. The Westminster village’s delusions about Britain’s importance in the world prevents them from accommodating this reality.”

Neutron generators are vital because they initiate nuclear explosions by bombarding plutonium or uranium with neutrons at a precise time, and they have to be regularly replaced. The neutrons are generated by fusing together two radioactive isotopes of hydrogen, tritium and deuterium.

The Scottish National Party’s Westminster leader and defence spokesman, Angus Robertson MP, has twice asked ministers to say where the neutron generators for Britain’s nuclear weapons were made, first in July 2004 and again in April 2005. On both occasions ministers withheld the information citing “defence, security and international relations”.

But a detailed report just released under freedom of information law by the US Government’s National Nuclear Security Administration has blown away British secrecy. It says that 14 neutron generators made at the Sandia National Laboratory in Albuquerque, New Mexico, were delivered to the UK in three batches in 2008.

When this was drawn to the attention of the MoD last week, it accepted for the first time that Trident’s neutron generators were made in the US. “Certain non-nuclear components of the ­existing warhead were procured from the US on cost-effectiveness grounds,” an MoD spokesman told the Sunday Herald. “These non-nuclear components include the neutron initiator.”

Robertson accused the UK Government of “misleading the public about the independence of the nuclear deterrent” as it had misled people about the Iraq war. “We simply cannot believe a word the Labour Government says – least of all when it comes to defence,” he said.

He was backed by the LibDem foreign affairs spokeswoman and East Dunbartonshire MP, Jo Swinson. “This news further undermines the idea of an independent British nuclear deterrent and demonstrates that it is increasingly reliant on the US,” she said.

“If Trident does not serve as an independent deterrent then there is little point in spending so much money on replacing it in its current form.”

John Ainslie, the co-ordinator of the Scottish Campaign for Nuclear Disarmament, pointed out that Trident warheads would become useless within a few years if the United States stopped supplying the neutron generators. “Successive defence ministers have kept this secret because they want to fool the public into believing that British nuclear weapons are independent,” he alleged.

“These secret purchases are a blatant breach of the Non Proliferation Treaty and a shameful example to the rest of the world. How can we tell other countries not to export nuclear bomb technology when we are making these black-market deals with the Pentagon?”

The MoD, however, argued that ­Britain’s bomb was “fully operationally independent of the US”. Said a ministry spokesman: “Only the Prime Minister can authorise the use of the UK’s nuclear deterrent, even if the missiles are to be fired as part of a NATO response.”