The Ministry of Defence is spending £2bn on new nuclear weapons plants before a formal decision has been taken over whether to replace Trident warheads, according to ministers.

The revelation has prompted fierce attacks on the MoD for making "a complete mockery" of the democratic process by pre-empting a decision and so attempting to force the hands of future governments.

The ministry says the investment helps to ensure the safety of the existing Trident warheads, but accepts that the money also maintains the capability to design a new warhead "should that be required".

Details of the MoD's investments have been unveiled for the first time. They include a £734m facility called Mensa for dismantling and assembling of warheads, a risky but essential maintenance process; a £634m highly enriched uranium plant called Pegasus; and a £231m high explosives factory called Circinus.

The plants are being built at the Atomic Weapons Establishment (AWE) sites at Aldermaston and Burghfield in Berkshire. Other facilities with similarly stellar names but smaller bills – Orion, Gemini, and Leo – are also being built as part of the AWE development plan covering 2005 to 2015. The costs of two more – Octans and Orchard – are being kept secret for commercial reasons.

The figures have been released by the defence minister Peter Luff in answer to a parliamentary question by the Green MP Caroline Lucas. Although a few were known from freedom of information requests or other sources, the bulk had previously been kept confidential.

"The fact that the MoD signed off on these costs before a decision has even been made on replacing the Trident warhead makes a complete mockery of the democratic process," said Lucas.

"It's clear that replacing this extravagant and discredited white elephant project would mean locking the UK into the costly technologies of the past, at a time when we should be developing the realistic defence solutions of the future."

She called on the government to end the "culture of secrecy" that allowed crucial military spending decisions to be pushed through without proper parliamentary scrutiny.

Spending so much on nuclear weapons at a time of economic austerity was also "morally indefensible and economically illiterate", she said.

The government decided in October last year to postpone a decision on whether to develop a new nuclear warhead until after the next election in 2015. To save money, and to appease the Liberal Democrats, ministers also deferred the main investment decision on replacing the submarines that will carry the warheads.

According to Peter Burt of the Nuclear Information Service, the new weapons plants would have a life of at least 40 years. "By spending billions of pounds now, the MoD is trying to force the hands of future governments into developing a new nuclear warhead, regardless of whether it will be necessary or affordable," he said.

An MoD spokeswoman pointed out that the government was committed to a "continuous at-sea nuclear deterrent" based on Trident. "This investment maintains the safety of the current Trident warhead stockpile by sustaining essential facilities and skills," she said.

"It also helps maintain the capability to design a replacement warhead should that be required following decisions in the next parliament."