A member of staff from the Health and Safety Executive (HSE) has lost a USB stick containing a safety assessment of a nuclear power plant.
The data was misplaced by an employee of the Office for Nuclear Regulation (ONR), an agency of the HSE, while attending a conference in India, sources say.
The lost stick contained a safety assessment of Hartlepool's nuclear plant which was undertaken in the wake of the Japanese radiation scare at the Fukushima nuclear plant.
An ONR spokesman said that there was no "significantly sensitive" data lost, and the majority of the report has since been put in the public domain.
He said: "An ONR member of staff lost an unencrypted USB pen drive containing the licensee's 'stress test' of a UK nuclear power station, which is a safety assessment of the station.
"It did not contain any significantly sensitive information.
"At the start of the European Commission 'stress test' programme, the licensees of all UK nuclear power stations committed to publishing their stress test reports, so most of the findings in this report are now in the public domain.
"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."
EU governments agreed last March that, in the wake of the Japanese earthquake and resulting tsunami which overwhelmed the Fukushima nuclear plant, all 143 of Europe's plants should undergo stress testing to common standards.