The US military has been using Britain's atomic weapons factory to carry out research into its own nuclear warhead programme, according to evidence seen by the Guardian.

US defence officials said that "very valuable" warhead research has taken place at the Atomic Weapons Establishment at Aldermaston in Berkshire as part of an ongoing and secretive deal between the British and American governments.

The Ministry of Defence admitted 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.

Last night, opposition MPs called for a full parliamentary inquiry into the extent of the collaboration at Aldermaston and campaign groups warned any such deal was in breach of international law. They added that it also undermined Britain's claim to have an independent nuclear weapons programme and meant British taxpayers were effectively subsidising America's nuclear programme.

The US president, Barack Obama, while on the campaign trail said he wanted to eliminate nuclear weapons and that one of his first actions on taking office would be to "stop the development of new nuclear weapons". But the Pentagon is at odds with the president. The defence secretary, Robert Gates, and other senior officials argue that the US's existing arsenal needs to be upgraded and that would not constitute "new" weapons.

Kate Hudson, of CND, said: "Any work preparing the way for new warheads cuts right across the UK's commitment to disarm, which it signed up to in the nuclear non-proliferation treaty. That this work may be contributing to both future US and British warheads is nothing short of scandalous."

Nick Harvey, defence spokesman for the Liberal Democrats, said parliament and the 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," he added.

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.

Referring 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, Harvey said: "There are some capabilities that the UK has that we don't have and that we borrow... that I believe we have been able to exploit that's been very valuable to us."

It is unclear whether the experiments are still being carried out but, in the same interview, Harvey 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). He said: "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."

Campaigners said the comments represent the first direct evidence that the US is using UK facilities to develop its nuclear programme. Lawyers acting on their behalf said the increasing levels of cooperation and the extension the MDA breach the non-proliferation treaty, which states: "Each nuclear weapon state party to the treaty undertakes not to transfer to any recipient whatsoever nuclear weapons or other nuclear explosive devices indirectly or indirectly."

The MoD admitted the two countries are working together, "examining both the optimum life of the UK's existing nuclear warhead stockpile and the range of replacement options that might be available to inform decisions on whether and how we may need to refurbish or replace the existing warhead likely to be necessary in the next parliament".

Congress has 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'."