A police officer faced disciplinary action after confidential files were stolen from the boot of his car.
The documents were taken while his car was parked outside a residential property and were handed in to police by a member of the public who found them dumped in a nearby street the following day.
An inquiry was launched by the Information Commisioner's Office (ICO) and it found Kent Police in breach of the Data Protection Act.
The ICO said the force's policies and procedures relating to the transportation and storage of personal information outside the office were "limited in scope and required further clarification".
It added that the officer had not used his secure briefcase to transport the papers, nor had he been provided with a secure storage facility at his home.
Kent Police has now signed a pledge to ensure staff whose roles give them access to sensitive information outside the office are provided with secure transportation and storage facilities.
There will also be clarification of the policies covering transportation, storage and use of personal and protectively marked information, the ICO added.
Sally-Anne Poole, enforcement group manager at the ICO, said: "It is essential that police forces ensure the correct safeguards are in place when storing and transferring personal information, especially when it concerns highly confidential information.
"A lack of awareness of data protection requirements can lead to personal information falling into the wrong hands."
Kent Police has not disclosed where the theft took place or the rank of the officer involved, but said in a statement he had "made a mistake".
It said: "He has been the subject of disciplinary action and has received further training regarding data protection. As a result of this incident, we have also re-examined our policies and procedures to ensure this does not happen again."
It is understood there was no impact on the incident to which the documents related.Reuse content