The UK has hired an extra 1100 scientists and technicians for its two nuclear weapons sites in Berkshire - an increase of 25% in less than four years - despite government denials that any decision has been taken on designing a new missile warhead for the Royal Navy.

Figures obtained by The Herald show that staff levels at the Atomic Weapons Establishment (AWE) at Aldermaston and the warhead assembly plant at nearby Burghfield rose from 3510 in 2004 to 4620 in February this year as part of a recruitment drive targeting physicists and researchers.

The government has also earmarked £5.7bn over the next three years for improvements at both sites and work to maintain the existing weapons stockpile, believed to be about 160 warheads.

Although Des Browne, the Defence Secretary, claims this will not have an effect on other military spending for the wars in Afghanistan and Iraq, it represents 5% of the entire budget this year and 5.25% in the subsequent two years.

While the government last year committed the UK to maintaining a submarine-based strategic deterrent which could cost up to £25bn to build and £65bn over its lifetime, it postponed final decisions on the choice and number of boats and the levels of firepower they will carry until the election of a new parliament after 2009.

Fifteen Scottish Labour MPs rebelled in a crucial vote on Trident in the Commons last March as Westminster backed plans to renew the nuclear weapons system by 409 votes to 161.

Sources as diverse as military officers and CND now believe the focus at Aldermaston is on creating a blueprint for a reliable replacement warhead as part of a secret project launched almost three years ago at the US nuclear laboratories in California and New Mexico.

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.

John Ainslie, Scottish CND's co-ordinator, said: "A lot of money and research is going into the design of the warheads, no matter what is said in parliament."

An MoD spokesman said no decisions on Trident's replacement have been taken and denied there was a programme to build a successor.

Clive Marsh, AWE's chief scientist, said: "The work splits into two main areas. The first is the requirement to maintain the current Trident stockpile. The second is to develop our overall warhead design."