They included renewables firms in a programme of espionage that “bears the hallmarks of state-sponsored operation”, according to digital security firm Symantec.

In May, the US Department of Homeland Security revealed that an “advanced hacking group” had compromised the control network of an unidentified public utility. A month later, the US traced dozens of hacking attempts on utilities and gas pipelines in 2012-13 to the Chinese army.

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