Estimates of the cost to clear up 12 of the UK’s Magnox nuclear power sites have increased by £2.7bn since 2017 and there remains “inherent uncertainty” over the final bill, the country’s public spending watchdog has warned.

In a report published on 11 September the National Audit Office also said the Nuclear Decommissioning Authority has incurred additional costs as a result of the failure of its original Magnox contract, with the NDA agreeing to pay up to £20m to exit the contract early.

The original contract was awarded in September 2014 to Cavendish Fluor Partnership (CFP). It was a 14-year contract to manage the decommissioning of two nuclear research sites and 10 Magnox sites.

In July 2016, the High Court ruled that the NDA had wrongly decided the outcome of the procurement process. In 2017, the NDA paid £97.3m to settle legal claims with another bidder and decided, based on legal advice, to terminate the contract with CFP nine years early.