Hazardous nuclear waste stored in run-down buildings at Britain's largest nuclear plant poses an "intolerable risk" to people and the environment, according to a British government watchdog.

The National Audit Office has said that for more than 50 years operators of the Sellafield power station in Cumbria failed to plan how to dispose of radioactive waste.

The plant is Britain's most hazardous nuclear site, storing enough high and intermediate level radioactive waste to fill 27 Olympic-sized swimming pools.

The report says 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.

Around 240 of Sellafield's 1,400 buildings are nuclear facilities and so far 55 buildings on the site have been decommissioned.