The government says it will preserve a “vital nuclear deterrent.”

Prospect has welcomed the decision and said the new facility was an important stage in the replacement Trident programme.

Negotiator Bob King said: “AWE is a major employer in the area, with hundreds of highly-skilled jobs. The fact that AWE is forging ahead with this new facility demonstrates its commitment to keep those jobs in the area.”

Project Pegasus will be constructed at Aldermaston to replace a facility built in the 1950s, which now falls short of modern safety design standards.

It will be used to store and handle enriched uranium components for nuclear warheads and reactor fuel for nuclear-powered submarines.

The Ministry of Defence initially refused to disclose the information due to commercial confidentiality, but the Information Commissioner ruled that it was in the public interest for the costs of the project to be released

The government has said it will not vote on the future of Britain’s current nuclear weapons system, Trident, until 2015.

In addition to the cost of Pegasus, £500m has also been allocated for Project Mensa at nearby AWE Burghfield to improve its warhead assembly facilities.

A MOD spokesman said Project Pegasus and Project Mensa could also be used for the current Trident warheads, which have a lifecycle that takes them to 2040, as well as for a theoretical replacement.

Trident’s submarines are expected to be retired in 2020.

MOD said: “This funding will ensure we maintain our commitment to providing our vital nuclear deterrent.

“It is necessary to invest in the facilities at AWE, which will provide assurance that the existing Trident warhead stockpile is reliable and safe.”