Sellafield's plans to replace ageing nuclear waste facilities posing significant risks to the population face considerable uncertainty, a public spending watchdog said.

Owners of the Cumbrian nuclear power station do not know how long it will take to build storage and treatment centres for the hazardous material or how much the final bill will be, according to the National Audit Office (NAO).

For more than 50 years, operators failed to plan how to dispose of the radioactive waste and some of the older facilities have deteriorated so much their contents pose significant risks to people and the environment, the report said.

Progress in 12 of the 14 major buildings and equipment projects considered "critical" for reducing risk, which range in cost from £21 million to £1.3 billion, failed to achieve what they were supposed to and had not provided good value for money, the NAO said.

Its report found there "is still considerable uncertainty in the schedules and costs" of the projects.

A long-term plan to clean up the Nuclear Decommissioning Authority-owned site, which is managed by Sellafield Limited, was agreed last year after an earlier one stalled because it was "unrealistic".

Sellafield, the UK's largest and most hazardous nuclear site, stores enough high and intermediate level radioactive waste to fill 27 Olympic swimming pools.

The highest risks are posed by the ponds and silos built during the 1950s and 1960s to store fuel for early reprocessing operations and radioactive waste, according to the report.

Margaret Hodge, who chairs the Public Accounts Committee, said: "Projects of this length and ambition are ripe for dithering and delay. I am dismayed to discover the clean-up of Sellafield is no different.

"My concern is that unless the authority holds Sellafield Ltd to a clear and rigorously benchmarked plan, timetables will continue to slip and costs spiral. It is unacceptable."