Sellafield Ltd has been fined more than £330,000 for failings in cybersecurity.

The firm, which operates the nuclear site in Cumbria, was charged after an investigation by the Office for Nuclear Regulation (ONR) found it failed to meet standards for protecting sensitive nuclear information, making its systems vulnerable to unauthorised access and loss of data.

The ONR did not find that any vulnerabilities had been exploited as a result of the failings.

On Wednesday at Westminster Magistrates’ Court, Chief Magistrate Paul Goldspring fined the company £332,500 and ordered it pay prosecution costs of £53,253, after it pleaded guilty at an earlier hearing to three counts of failing to comply with its approved security plan, a spokesman for the ONR said.

Paul Fyfe, the ONR’s senior director of regulation, said: “It has been accepted the company’s ability to comply with certain obligations under the Nuclear Industries Security Regulations 2003 during a period of four years was poor.

“Failings were known about for a considerable length of time but despite our interventions and guidance, Sellafield failed to respond effectively, which left it vulnerable to security breaches and its systems being compromised.

“Nevertheless, with new leadership and additional resources in place at Sellafield Ltd, we have seen positive improvements during the last year, and evidence the senior leadership is now giving cybersecurity the level of attention and focus it requires.

“We will continue to apply robust regulatory scrutiny where necessary to ensure all risks, including cybersecurity, are effectively managed by the nuclear industry.”

Energy Secretary Ed Miliband said: “We take the safety of our nationally significant infrastructure very seriously and I welcome the fact we have a robust regulator holding our nuclear industry to account.

“I have written to the CEO of the Nuclear Decommissioning Authority seeking assurance that cybersecurity failings at Sellafield are being addressed and cannot happen again.”