The UK’s most hazardous nuclear site, Sellafield, has been hacked into by cyber groups closely linked to Russia and China, the Guardian can reveal.

The astonishing disclosure and its potential effects have been consistently covered up by senior staff at the vast nuclear waste and decommissioning site, the investigation has found.

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 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.