BRITAIN has been secretly designing a new nuclear warhead in conjunction with the Americans, provoking a legal row over the proliferation of nuclear weapons.

The government has been pushing ahead with the programme while claiming that no decision has been made on a successor to Britain’s Trident nuclear deterrent. Work on a new weapon by scientists at the Atomic Weapons Establishment at Aldermaston in Berkshire has been under way since Tony Blair was re-elected last May, and is now said to be ahead of similar US research.

The aim is to produce a simpler device using proven components to avoid breaching the ban on nuclear testing. Known as the Reliable Replacement Warhead (RRW), it is being designed so that it can be tested in a laboratory rather than by detonation.

“We’ve got to build something that we can never test and be absolutely confident that, when we use it, it will work,” one senior British source said last week.

The secret programme to build a new warhead in close co-operation with the Americans will spark anger among Labour opponents of any replacement of the Trident programme, which is estimated to have cost nearly £10 billion.

Developing a new weapon would also, according to expert advice from Cherie Booth’s Matrix chambers, be a material breach of the Nuclear Non-Proliferation Treaty. The office of Lord Goldsmith, the attorney-general, refused to comment on whether it had been asked for legal advice by No 10.

Both Labour backbenchers and the Liberal Democrats accused the government of introducing a replacement nuclear weapon by the back door without a parliamentary debate.

Paul Flynn, a Labour backbencher who has drafted parliamentary motions questioning the need for a Trident replacement, insisted there had to be a proper debate. “The Trident missiles will last for another 20 years,” he said. “Who on earth are we going to take on with them anyway? Replacing them wrecks any standing we have when we preach non-proliferation to countries like Iran.”

Michael Moore, the Lib Dem foreign affairs spokesman, called for a statement. “This work would appear to pre-empt the proper debate the prime minister has promised,” he said.

The controversy is set to ignite this week with an embarrassing report by the Foreign Policy Centre (FPC), set up under Blair’s patronage, calling for Trident to be scrapped and not replaced.

On Tuesday the defence select committee will take evidence from experts, most of whom are expected to say that there is no need for a new nuclear deterrent.

The FPC report says that Britain’s independent deterrent is an illusion. The missiles are stored in the United States and have to be collected by a British submarine before it goes on patrol.

Aldermaston is run by a consortium headed by Lockheed Martin, a US company, and there are 92 Americans working there, including the managing director and four of his senior managers.

“The UK should cease to try to keep up appearances and adopt a policy based on the reality that it is not an independent nuclear power,” the FPC report concludes. “Trident should not be replaced and should be phased out now.”

Blair is said to want to decide on Trident’s replacement before he steps down. “It is a huge decision for the country and it will probably be done in a far more open way than the decisions have been taken before,” he said last month.

As he spoke, work was well advanced at Aldermaston on designs for the RRW. The US Los Alamos and Lawrence Livermore nuclear laboratories began a competition to produce an RRW last May. But Washington sources say the British have been designing their own Reliable Replacement Warhead and “are now ahead of the Americans”.

One possible way to avoid breaching the Nuclear Non-Proliferation Treaty is for Blair to announce that the new deterrent will have fewer warheads. We currently have about 200.

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