Greenpeace say that scrapping Trident will save billions
Scrapping Britain's nuclear deterrent would save the government substantially more than expected, Greenpeace says.
A report drawn up by the pressure group says replacing Trident would cost about £34bn, at least twice what the government estimates.
The government says replacing the nation's nuclear deterrent would cost between £15bn and £20bn.
The Ministry of Defence said it was impossible to predict the material costs of replacing Trident.
Greenpeace claim the government's figures exclude additional costs, such as VAT, exchange rate fluctuations or the price of renewing some new missiles.
It also prices the lifetime running costs of Trident at around £63bn, which it says is money that could be saved over the next 30 years.
Greenpeace's executive director John Sauven described the expense as "quite simply astronomical".
He said: "These new figures have been revealed at a time when many defence experts are raising serious questions over the system's strategic value, while the need to cut government spending is obvious to everyone.
"Any government which renews Trident would be wasting £100bn and a rare and precious opportunity to make the world a safer place, just as the Obama administration is making real progress on multilateral disarmament. It doesn't make sense on any level."
A recent report from think tank Reform suggested that not renewing Trident could save £70bn over 25 years.
A further £40bn could be trimmed off the defence budget by scrapping some hardware and reducing waste, it added.
But earlier this week Business Secretary Lord Mandelson said scrapping Trident would save less money than expected.
And Defence Secretary Bob Ainsworth has said the UK would "stick" with Trident but would look at its costs.
The Conservatives back Trident but say it will form part of an overall review of the defence budget which they plan immediately if they win the next election.
The Lib Dems have called for a smaller, cheaper deterrent to reflect the changing nature of warfare and threats to the UK.
Matthew Knowles, a spokesman for the aerospace, defence and security trade body ADS, said: "Proposing cuts in our defence budget is short-sighted and very risky in an unpredictable, multi-polar world.
"Further, defence expenditure can and should be used as a path out of recession via the 300,000 people who work in the industry across the UK... Cutting defence will cut our economic recovery."
Several senior Labour figures, including Charles Clarke, have called for Trident not to be renewed.