The Ministry of Defence (MOD) has been accused of making "avoidable mistakes" that have cost the UK taxpayer over £1 billion more than originally planned.
A new report from the Public Accounts Committee (PAC) said a series of key projects to upgrade infrastructure, which supports Britain's nuclear deterrent, were running years behind schedule and £1.35 billion over budget.
The report said the MOD has failed to learn from previous errors over the last 30 years, describing it as "unacceptable".
“The department cannot explain why its leadership has not ensured that it learned from these experiences," the report said.
The MOD told the committee it "immensely regrets" the amount of money lost but warned costs could continue to increase.
The report found the infrastructure projects faced delays of between 1.7 and 6.3 years.
It said Project Mensa, a programme to build a new nuclear warhead assembly and disassembly facility at the Atomic Weapons Establishment site in Burghfield, faced almost £400 million in extra costs almost immediately after construction began before designs were finalised.
The other projects reviewed in the report included the new core production capability at Rolls Royce's Raynesway site and the primary build facility at BAE Systems' shipyard at Barrow-in-Furness.
Meg Hillier MP, Chair of the PAC, said: “To utterly fail to learn from mistakes over decades, to spectacularly repeat the same mistakes at huge cost to the taxpayer – and at huge cost to confidence in our defence capabilities - is completely unacceptable.
“The Department knows it can’t go on like this, it knows it must change and operate differently. The test now is to see how it will do that, and soon."
The PAC said it expects a 2020 report from the department later this year, including a progress assessment on its use of taxpayer money.
A Ministry of Defence spokesperson told Forces News: "The MOD is carefully examining the conclusions and recommendations set out in the PAC report and will respond formally in due course.
"Nuclear infrastructure projects are vastly complex and necessarily bespoke in design, but we continue to work closely with the regulators and our industry partners. Together, we are committed to strengthening the management of nuclear programmes, including significant investments in infrastructure to store and update weapons."