This is up almost 19 percent on the previous official estimate in 2019 surrounding the "ageing" and "contaminated" site on England's northwest coast, the NAO said in a statement.

"Ongoing concerns over project management, the pace of delivery and staffing at Sellafield -- the UK's most complex and challenging nuclear site -- mean it is not yet achieving value for money," the Office added.

Sellafield stopped generating electricity in 2003, while Britain's new Labour government plans to further decarbonise the country's energy mix.

Prime Minister Keir Starmer's government launched its flagship green energy plan following Labour's election win in early July, with the creation of a publicly-owned body to invest in offshore wind, tidal power and nuclear power.

Sellafield is owned by the Nuclear Decommissioning Authority (NDA), described as a non-departmental body sponsored by the Department of Energy Security and Net Zero.

According to the NAO, Sellafield "holds a legacy of contaminated buildings, untreated waste and ageing facilities".

It added in its latest update: "While workers at Sellafield have started retrieving and safely storing waste, the NDA expects full site remediation will take until 2125."