Scientists are secretly working on the design of a revamped British nuclear warhead.

The new device, designated the High Surety Warhead is understood to be under development at the Atomic Weapons Establishment at Aldermaston in Berkshire.

The top-secret project is being run in conjunction with US efforts to build a range of modernised "failsafe" nuclear firepower for its own submarine-launched Trident missiles.

The Campaign for Nuclear Disarmament (CND) claims the research is a clear and cynical breach of international disarmament and non-proliferation treaties.

News of the research has leaked out less than a year after a succession of UK ministers denied plans to upgrade or refurbish the Royal Navy's existing stockpile of warheads, believed to number about 160 weapons.

Des Browne, the Defence Secretary, said in a Commons reply quoted by Hansard on December 18, 2006, that no such changes were likely "in the next five years". He then added that "decisions on whether and how we may need to refurbish or replace the warhead stockpile are likely to be necessary in the next parliament".

Insiders say the new warhead is the UK version of the Reliable Replacement Warhead programme which started more than two years ago at the US military's California and New Mexico nuclear laboratories.

The aim is to produce warheads which contain fewer degradable components, giving them a longer shelf-life, and to make them so dependable that none would have to be detonated in an underground explosion that would contravene the worldwide test ban in place since 1998.

The UK is meanwhile in the process of investing almost £2.2bn in the Aldermaston site to equip it with a state-of-the-art Cray supercomputer codenamed Larch and a laser codenamed Orion to help model nuclear explosions in place of live testing. The supercomputer, bought with £20m of taxpayers' money, is so fast that the six billion inhabitants of the planet would have to make 7000 calculations each per second to keep pace with it.

A programme to hire scientists, physicists and engineers is also under way at the 750-acre site. Recent recruitment adverts included the post of "warhead electrical engineers".

John Ainslie, Scottish CND's co-ordinator, said yesterday: "Dr Strangelove is alive and well and working at Aldermaston. A lot of money and research is going into the design of the warheads, no matter what is said in parliament. Everything is geared towards making the weapons more reliable and more accurate. That contravenes every aim of the nuclear non-proliferation treaty. It's impossible to be working towards the goal of nuclear disarmament while you're in the business of producing better ones for your own forces."

An MoD spokesman said: "No decisions on any replacement for Trident have yet been taken and there is no programme to build a successor warhead."