A memory stick containing a safety assessment of a nuclear power plant in north-east England has been lost by an official, it has emerged.
The unencrypted USB pen drive, containing a 'stress test' safety assessment of the Hartlepool plant, was lost by an employee of the Office for Nuclear Regulation (ONR).
The ONR said the key did not contain "significantly sensitive" data.
It said an internal investigation had begun.
The ONR, which is an agency of the Health and Safety Executive, said the use of unencrypted devices for transporting documents with a security classification was not allowed.
So-called stress tests were carried out at European power plants following the Japanese earthquake which affected the Fukushima nuclear power plant.
Published on website
An ONR spokesman said at the start of the stress test programme, there had been a commitment to publishing the reports, so most of the findings were now in the public domain.
But he said: "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."
A spokeswoman for EDF Energy, which operates the Hartlepool plant, said the lost key did not contain any significantly sensitive information.
She said reports had been published on its website in January.
"The reports on the website have slight differences to the full technical versions in that some of the technical language has been simplified to avoid misinterpretation of the information, and more detailed data like site layouts has been removed although we reiterate that the document on the memory stick does not contain any significantly sensitive information."