So, minister, are we developing new nuclear weapons or not?

The Government was accused last night of covertly beginning work on a new nuclear warhead, despite ministers' assurances that no decision on replacing the Trident nuclear deterrent had been made.

The chief scientist at Aldermaston, the UK's top-secret atomic weapons facility, has told potential recruits that "most of our research" is devoted to "the ability to provide a new warhead". In a video link, aimed at recruiting top scientists, Dr Clive Marsh lets slip that scientists at Aldermaston are busy working on the development of "our overall warhead design and assurance capabilities".

His remarks were yesterday seized on by anti-nuclear campaigners, who claim that Tony Blair's promise to have a debate on whether to replace Trident is a "sham". They say ministers have misled Parliament by claiming that no work on a new nuclear warhead is being done and have accused them of breaking and the Non-Proliferation Treaty on nuclear weapons.

Des Browne, the Secretary of State for Defence, told the House of Commons in reply to a question from an MP in May that "the Atomic Weapons Establishment [AWE] is not engaged in the development of any new warheads".

However, according to Blake Lee Harwood, Greenpeace's campaign director: "These revelations from Aldermaston's top scientist prove that Tony Blair's promised debate was a sham. The Government is pretending to consult but it has already given the nod to a new weapons system costing billions of pounds. In a single move, Tony Blair has broken the Non-Proliferation Treaty and his promise to the country."

In a webcast on the Aldermaston internet site, Dr Marsh says that Aldermaston's hi-tech research "splits into two main but inter-related areas".

"The first is the requirement to maintain the existing Trident nuclear stockpile," he says. The second, which accounts for most of Aldermaston's research, is related to maintaining the ability to carry out warhead design. He says research is ongoing on "our overall warhead design and assurance capabilities, including the ability to provide a new warhead lest our Government should ever need it as a successor to Trident. He adds: "Most of our research is conducted in this area."

The scientist goes on to explain that research is also being conducted on developing sophisticated computer modelling to test the safety of models and designs. He said that the ban on nuclear testing means that computers now have to be used "to validate those aspects of the models that are accessible in the laboratory" and "to consolidate the accuracy of our predictive capabilities".

His remarks appear to conflict with those of ministers, who have said that there will be a debate on whether to replace Trident before any work proceeds. The Ministry of Defence denied that the Atomic Weapons Establishment was working on designing new warheads, but said it was maintaining the capacity to do so and to test them.

"There is no programme to design or develop a new warhead. The explanation for this is reiterating the ability to be ready to design something if needed . . . They are up to date but are not designing or developing a new warhead. No decision has been taken in principle or in detail," an MoD spokesman said.

Many MPs believe that Mr Blair has already made up his mind to replace the Trident nuclear weapons system with a more advanced nuclear deterrent. The Government is expected to indicate by the end of this year whether the warhead will be replaced.

Labour MPs were furious that motions on the future of Britain's nuclear deterrent were blocked at their party conference in Manchester last month. Suspicions that Aldermaston has already been given the green light by Downing Street to begin work on a new warhead have been fuelled by a huge expansion at the site.

The secret research centre is spending millions of pounds on powerful lasers capable of testing new nuclear technology. The site is also being expanded in a massive construction project. Aldermaston's recruitment drive has already created jobs for physicists, engineers and technicians. It has plans to recruit hundreds more staff.

The Government has insisted the extra staff are being hired to maintain the current Trident system. Aldermaston declined to comment.