Highly dangerous plutonium canisters are “decaying faster than anticipated” at the Sellafield nuclear plant and present an “intolerable risk” if they started to leak, the spending watchdog has warned.

Government scientists have now agreed to spend an extra £1billion to make them safe by wrapping them in packaging, the National Audit Office said today.

Britain has the largest amount of civil plutonium – a bi-product of nuclear fuel reprocessing – in the world, around 40 per cent of the global total.

Most of the plutonium is stored at Sellafield in Cumbria, where it is managed by the Nuclear Decommissioning Authority (NDA).

The problems have occurred because some of the plutonium canisters are judged to be “unsuitable” for storage in a new facility which only opened in 2012, the NAO said.

Staff are now racing against the clock to build a new £1.5billion facility – and are having to make contingency plans for the next two years while the new depot is constructed.