The cost of cleaning up Sellafield in northwest England is expected to increase by £21bn (€25bn, $27bn) to £136bn and the UK’s “most complex and challenging nuclear site” cannot show how it offers taxpayers value for money, the public spending watchdog has said.

The National Audit Office (NAO) says in a report released on 23 October that ongoing concerns over project management, the pace of delivery and staffing at the state-owned site mean it is not yet achieving value for money.

It said large projects are being delivered later than planned and at higher cost, alongside slower progress in reducing multiple risks.

“Continued underperformance will mean the cost of decommissioning will increase considerably, and ‘intolerable risks’ will persist for longer,” said Gareth Davies, head of the NAO.