GCHQ cyber experts have been called in after a digital attack on a major player in Britain’s nuclear power ­industry triggered a security crisis.

The National Cyber Security Centre (NCSC), an arm of GCHQ, has been ­secretly providing assistance to a ­nuclear power company in the UK that has struggled to recover after being hit by a cyber attack, The Telegraph can reveal.

A Nuclear Decommissioning Authority (NDA) report, obtained using freedom of information legislation, said officials are “aware that an important business in the Nuclear Power Generating Sector has been negatively impacted by a cyber attack and has had to rely on expertise from the NCSC to help them with recovery”.

The document, from a board committee meeting dated March 13 2019, is believed to be the first time evidence of a successful cyber attack on a nuclear company in the UK has come to light.

It is not known what damage the ­attack caused or whether the attack put public safety at risk. The NCSC ­declined to name the company ­involved or give details about the ­attack citing “operational sensitivities”.

The NDA, which manages the clean-up of old nuclear plants and spent fuel, said it would be “inappropriate” to provide details on grounds that “the event relates to an organisation that is not part of the NDA group”.