A proposed new enriched uranium facility at the Atomic Weapons Establishment (AWE) in Berkshire will cost £750m, it has been revealed.
The costs of Project Pegasus were given to Nuclear Information Service (NIS) following a request under the Freedom of Information Act.
NIS, which aims to foster debate on nuclear disarmament, has criticised the cost of the facility that will be used to handle and store nuclear weapons components.
However, the government says it will preserve a "vital nuclear deterrent".
A spokesman from Prospect, the union that represents over 1,200 engineers and scientists at AWE, said the new facility was an important stage in the Trident program.
He said: "AWE is a major employer in the area, creating hundreds of highly skilled jobs.
"I'd rather have a modern, safe and efficient uranium enrichment unit than an old and less efficient unit."
Project Pegasus will be constructed at Aldermaston to replace a facility built in the 1950s, which now which 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.
Costings from the Ministry of Defence (MoD) now show the Aldermaston facility was priced at £750m in 2007.
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
Peter Burt, project director of NIS, said he was concerned such large sums of money were being spent on nuclear weapons.
The government has said it will not vote on the future of Britain's current nuclear weapons system, Trident, until 2015.
Mr Burt said: "At a time when the defence budget is facing severe cuts to the armed forces, why are we spending money on nuclear weapons?
"After the Cold War, they bear no relevance for the security problems we face."
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 spokesman from the MoD 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.
It is envisaged any successor to Trident could use the existing warheads.
In a statement, the MoD said: "This funding, which includes Project Pegasus, was announced six years ago and 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."