Leeds Council has apologised for losing a memory stick containing unencrypted details of 5,000 nursery-age children.
The council originally believed the stick, which was found by a member of the public, contained no sensitive data.
It was not encrypted or protected by a password. It contained names, addresses, dates of birth, phone numbers, child protection information, ethnicity and whether their parents are claiming benefits.
The council is writing to parents apologising for the leak.
A Leeds City Council spokeswoman, said: “We take issues of information security very seriously and are very sorry that this breach has occurred. We have guidance in place which seeks to prevent such incidents occurring including advice on using memory sticks.
“The loss was immediately reported by the employee concerned to their line manager and enquiries were made to recover it. Regrettably it could not be located. At the time, it was understood that no sensitive or confidential data was on this stick, so no further action was taken.
“Unfortunately, once recovered, it became apparent the memory stick did have sensitive information on it that should not have been there.
“As soon as we were made aware of the content, a full investigation into the circumstances of this case was launched and an immediate reminder to all staff is being issued regarding the security of personal and sensitive information.
“We are grateful to the member of the public who found and returned the memory stick.”
The council could not explain how, or why, the information was put on the memory stick in the first place. ®