Sellafield nuclear site hacked by groups linked to Russia and China

But sources said breaches were first detected as far back as 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.

The site has the largest store of plutonium on the planet and is a sprawling rubbish dump for nuclear waste from weapons programmes and decades of atomic power generation.

Guarded by armed police, it also holds emergency planning documents to be used should the UK come under foreign attack or face disaster.