Opposition MPs Monday were calling for a full parliamentary inquiry into reports that the US has been carrying out illegal nuclear arms research in the UK.
According to the evidence seen by the Guardian newspaper, the US military has been using the Atomic Weapons Establishment in southern England for "very valuable" warhead research as part of an ongoing and secretive deal between the British and American governments.
The Ministry of Defence was said to have confirmed it is working with the US on the UK's "existing nuclear warhead stockpile and the range of replacement options that might be available" but declined to give any further information.
MPs were seeking to find out the extent of the collaboration, while campaign groups warned any such a deal was in breach of international law and undermined Britain's claim to have an independent nuclear weapons programme.
The Liberal Democrat’s shadow defence secretary Nick Harvey, said the parliament and country would react with "outrage" at the prospect of British taxpayers funding a new US nuclear weapon.
"All this backroom dealing and smoke and mirrors policy is totally unacceptable, the government must open the Aldermaston accounts to full parliamentary scrutiny," Harvey said.
Britain’s veteran CND campaign group accused the UK of breaching the nuclear Non-Proliferation Treaty, saying that any work preparing the way for new warheads “cuts right across the UK's commitment to disarm.”
“That this work may be contributing to both future US and British warheads is nothing short of scandalous," said the chair of CND Kate Hudson.
The Guardian said that the extent of US involvement at Aldermaston came to light in an interview with John Harvey, policy and planning director at the US National Nuclear Security Administration, carried out last year by the thinktanks Chatham House and the Centre for Strategic Studies.
Harvey referred to "dual axis hydrodynamic" experiments which, with the help of computer modelling, replicate the conditions inside a warhead at the moment it starts to explode.
"There are some capabilities that the UK has that we don't have and that we borrow,” Harvey said. “I believe we have been able to exploit that's been very valuable to us,” he said.
He admitted that the US and UK had struck a new deal over the level of cooperation, including work on US plans for a new generation of nuclear warhead known as the Reliable Replacement Warhead (RRW).
"We have recently, I can't tell you when, taken steps to amend the MDA [Mutual Defence Agreement], not only to extend it but to amend it to allow for a broader extent of cooperation than in the past, and this has to do with the RRW effort," Harvey said.
The US Congress has already stopped funding research into RRW but campaigners believe the US military may have used facilities in the UK to get around the restrictions at home.
"Billions of pounds have been poured into the Atomic Weapons Establishment over recent years to build new research facilities," said Hudson.
"If these are being used to support US programmes outside Congress's controls on spending, it raises even more serious questions about why the British taxpayer is paying for a so-called 'independent deterrent'," she said.