The cost of maintaining three nuclear sites spiralled by £1.35bn because of poor management by the Ministry of Defence (MoD), Whitehall’s watchdog has found.

The National Audit Office has also said that work to complete the building and maintenance of the UK’s Trident nuclear deterrent in Cumbria, Derbyshire and Berkshire will also be completed more than six years later than planned.

In a damning conclusion to a report released on Friday, auditors said the MoD has failed to learn from past mistakes while upgrading the UK’s nuclear defence capabilities.

Civil servants have relied on the work of three monopolistic suppliers – the Atomic Weapons Establishment (AWE), Rolls Royce and BAE Systems – which means that contracts cannot be terminated if poor performance is identified, the report said.

Nearly half of the extra costs – £647m – has been put down to the revision of work which was launched without proper plans.

The findings follow confirmation that Dominic Cummings is seeking to launch a review of the MoD’s spending strategy following concerns that it has wasted billions over misjudged procurement.

The ministry was given a £2.2bn increase in funds in September’s spending review. In February, parliament’s public accounts committee identified a £7bn blackhole in its 10-year plan for equipment.

Auditors examined three programmes within the nuclear enterprise programme to replace ageing facilities, some dating back to the 1950s.

They were the construction of a new nuclear warhead assembly facility in Burghfield, Berkshire; the erection of a new nuclear core production facility in Raynesway, Derbyshire; and the construction of a facility at the BAE shipyard in Barrow-in-Furness.

While the three projects were valued at £2.5bn, auditors found that costs had risen by £1.35bn with delays running at between 1.7 and 6.3 years.

The MoD was continuing to repeat mistakes made in the last cycle of investment in the nuclear enterprise in the 1980s and 1990s, the report found.

They included “starting to build before requirements or designs were sufficiently mature, increasing risks through inappropriate contracts and failing to engage with regulators to understand requirements”.

Similar problems had also been encountered in UK civil nuclear projects and US defence programmes, but the NAO said it had found no evidence of lessons being shared or learned.

Among the specific problems highlighted, auditors said delays to project Mensa – the Burghfield scheme – had meant the MoD had to spend an additional £21m to keep existing facilities going.

Some of the work was subjected to specific and expensive designs, sometimes referred to as “gold-plating”, the report said. This forced costs up but could not be challenged because the MoD and the site operators struggled to recruit staff able to challenge complex demands by regulators.

“Throughout our study, the department, regulators and site operators all considered it had occurred and remained a potential risk,” it said.

Gareth Davies, the head of the NAO, said: “While these infrastructure projects are complex, the MoD has encountered similar challenges before in its nuclear work.

“Although it has recently introduced changes to enhance its oversight of the projects and improve its contracts with suppliers, it should have learnt earlier from past mistakes and the experience of others in the nuclear sector.

“Instead, the MoD’s failure to mitigate commercial and delivery risks early on has led to project delays and cost increases, as well as impacting its wider work.”

An MoD spokesperson said: “As the National Audit Office has acknowledged, nuclear infrastructure projects are often large and complex with niche designs. We are carefully examining the conclusions in their report and will respond formally in due course.”