The Guardian had earlier reported that Sellafield in northwest England, which carries out nuclear fuel reprocessing, nuclear waste storage and decommissioning, had been hacked into by cyber groups closely linked to Russia and China.

"Our monitoring systems are robust and we have a high degree of confidence that no such malware exists on our system," the government said. "This was confirmed to the Guardian well in advance of publication, along with rebuttals to a number of other inaccuracies in their reporting."

(Reporting by Muvija M; editing by Michael Holden)