Kent Police has been fined £100,000 after highly sensitive interview tapes were found abandoned in the basement of a former police station in Gravesend.
Other confidential information, including records going back to the 1980s, was left at the Windmill Street site after the building was fully vacated in July 2009 - a year after officers moved to the new North Kent Police Station in Thames Way, Northfleet.
The information was discovered when a police officer was visiting a business owner about an unrelated matter on November 27, 2012 and noticed a pile of tapes with the Kent Police logo on them.
The business owner confirmed he had found the tapes in the basement of the old police station, after purchasing the site two months before, and was planning on watching them for entertainment.
The force has now been fined by the Information Commissioner’s Office for what head of enforcement Stephen Eckersley called a "clear lack of oversight".
He 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."
Hundreds of documents and evidence tapes were recovered including recorded interviews with informants, crime victims and individuals who had subsequently been convicted.
The documents also included information about police staff.
The ICO’s investigation found Kent Police had no guidance or procedures in place to makes sure personal information was securely removed from former premises.
A Kent Police spokesman said: "It is unacceptable for police property to have been left at the site of the former station in Gravesend following the move to North Kent Police Station in July 2008.
"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."