Replacing Trident, the U.K.'s nuclear weapons system, will cost "at least" £205 billion ($295 billion), according to the Campaign for Nuclear Disarmament (CND).
In an announcement on Thursday, the CND said that the cost had doubled since 2014. The Trident system is in the Royal Navy's Vanguard Class submarines, which are set to be replaced by four new "Successor" class submarines.
"For too long the pro-Trident lobby has been in denial about the real cost to our economy of Trident replacement," Kate Hudson, general secretary of the CND, said in a statement.
"These new calculations, drawn from actual government figures, show that the bill has spiralled beyond all expectations," Hudson added.
The government describes the replacement plan as equal in scale to huge infrastructure projects such as railway scheme Crossrail, which is projected to cost £14.8 billion.
The first Successor submarine is set to enter service in the "early 2030s" according to the government's National Security Strategy and Strategic Defense and Security Review 2015.
The government estimates that it will cost £31 billion to manufacture the four Successor submarines alone. To put this in context, the U.K's defense spending in 2015-16 is estimated to be £45 billion, according to the Office for Budget Responsibility.
The CND's figures state that, among other things, replacing warheads would cost £4 billion, with in service costs amounting to £142 billion.
"£205 billion of public money is a huge amount. Pouring it into a nuclear weapons system that experts say could be rendered obsolete by new technology is hardly a wise choice," the CND's Hudson added.
"Far better to spend it on industrial regeneration, building homes, tackling climate change or meeting our defense needs in usable ways," she said.
The U.K. government says its continuous at sea deterrent has been in place for 46 years. Defense secretary Michael Fallon has previously described the U.K.'s nuclear deterrent as providing "the ultimate guarantee of our security and our way of life."
Responding to the CND figures, the U.K.'s minister for defense procurement, Philip Dunne, described them as "more fiction dressed up as fact from known opponents of the strategic deterrent."
"We do not recognize these figures. We have been clear on the cost estimates published on the Successor submarine," Dunne added.
"We are replacing the submarines and that cost equates to 20 pence in every £100 of annual government spending. The in-service costs remain unchanged - around 6 percent of the Annual Defence budget."
On Friday, a report from the Royal United Services Institute (RUSI) said that the Ministry of Defence's budget would increase by one percent "in real terms over the next four years." Spending on both counter-terrorism and cyber security would increase, RUSI said, with security and intelligence agencies benefiting from a 16.7 percent real-terms increase.