According to the Guardian the hack and its potential effects have been consistently covered up by senior staff at the vast nuclear waste and decommissioning site.

The Guardian has discovered that the authorities do not know exactly when the IT systems were first compromised. But sources said breaches were first detected in 2015 when experts realised sleeper malware -- software that can lurk and be used to spy or attack systems -- had been embedded in Sellafield's computer networks.

It is still not known if the malware has been eradicated. It may mean some of Sellafield's most sensitive activities, such as moving radioactive waste, monitoring for leaks of dangerous material and checking for fires, have been compromised.

Sources suggest foreign hackers have likely accessed the highest echelons of confidential material at the site, which sprawls across 6 sq km (2 sq miles) on the Cumbrian coast and is one of the most hazardous in the world.