An official at the Health and Safety Executive (HSE) has lost an unencrypted USB stick containing data relating to one of the UK’s nuclear power stations.

The data came from a stress test of the Hartlepool nuclear power station in North East England. Contrary to HSE policy it was placed on an unencrypted USB stick, and subsequently mislaid at a nuclear energy conference in India.

"Most of the findings in this report are now in the public domain," the Office for Nuclear Regulation (an arm of the HSE) said in a statement.

"The use of unencrypted USB pen drives is not permitted by ONR for transporting documents with a security classification. An internal investigation has been undertaken by ONR."

Stress tests were ordered for all ten of the UK’s nuclear power stations in the wake of the Fukushima disaster in Japan.

The data loss is revealed as David Cameron and French president Nicolas Sarcozy prepare to announce an Anglo-French alliance on nuclear power projects and safety initiatives.