A police force has been fined £100,000 after highly-sensitive information, including interview tapes with informants and victims, were abandoned at a former police station.
Records dating back to the 1980s were left in the basement of the old station after it is believed the site was vacated by Kent Police in July 2009.
These included recorded police interviews with informants, crime victims and people who were later convicted of offences.
Some documents also related to police staff.
The Information Commissioner's Office (ICO), which levied the fine, said only "good fortune" prevented the files falling into the wrong hands.
The bungle emerged when a police officer visiting a business owner about an unrelated matter noticed a pile of tapes with the Kent Police logo on them on November 27 2012.
The business owner confirmed he had found the tapes in the basement of the police station, after buying the site two months earlier, and that he planned to watch them for entertainment.
The next day, the force visited the old station site and recovered hundreds of extra documents and evidence tapes, including recorded interviews with informants and victims.
An inquiry by the ICO found Kent Police had "no guidance or procedures in place to make sure personal information was securely removed from the former premises".
It said that the problem was made worse due to an apparent breakdown in communications between various departments involved in the move.
Stephen Eckersley, the ICO's head of enforcement, said: "If this information had fallen into the wrong hands, the impact on people's lives would have been enormous and damaging.
"These tapes and files included extremely sensitive and confidential information relating to individuals, many of whom had been involved in serious and violent crimes.
"How a police force could leave such information unattended in a basement for several years is difficult to understand.
"Ultimately, this breach was a result of a clear lack of oversight, information governance and guidance from Kent Police which led to sensitive information being abandoned.
"It is only good fortune that the mistake was uncovered when it was and the information hasn't fallen into the wrong hands."
Kent Police said they accepted the findings of the ICO and that it was "unacceptable" for police property to have been left behind at the former station in Gravesend.
A police spokesman said: "Since this was reported in 2012, Kent Police has reviewed its policies to ensure all documentation and files containing personal information are dealt with appropriately and in compliance with data protection legislation.
"In addition, we have now implemented new procedures when vacating police premises.
"After the discovery of the loss of data from the former police station, officers quickly retrieved and secured all documentation and property belonging to Kent Police.
"No sensitive information was lost or further disseminated."