Greater Manchester Police has revealed that an officer had an unencrypted USB device containing detailed information on active operations and personal information on members of the public stolen from his home.
The force said that it has started contacting those affected, but Assistant Chief Constable Garry Shewan sought to allay any fears that the device held information on police informants.
"I want to make it clear that any suggestion this memory stick contained lists of names of people who 'tipped off' police is inaccurate," he said.
"We know what is on the device because the officer has told us, and so far we have only had to speak to one member of the public whose personal information may have been compromised."
Shewan added that the force is working to ensure that such incidents do not happen again, and that staff are expected to be aware of procedures relating to the handling of personal information.
"Greater Manchester Police is reinforcing to officers and staff the importance of keeping information secure at all times. We expect all officers and staff to adhere to rigorous standards with regard to confidential and sensitive documents," he added.
"We are also conducting a thorough review of how we store information to ensure this does not happen again."
The Information Commissioner's Office (ICO) confirmed that it has been made aware of the breach, and is taking the appropriate steps.
"We have recently been informed of a possible data breach which may involve Greater Manchester Police," a spokesperson said.
"We will be making enquiries into the circumstances of the alleged breach of the Data Protection Act before deciding on the necessary course of action."
The ICO recently chided Lancashire Police Authority for publishing the details of an individual's complaint on its web site and failing promptly to remove the information when informed of the mistake.
V3 reported on Tuesday that a Freedom of Information request to the ICO revealed that a police force was one of several organisations to turn down a free audit of data protection practices by the watchdog.
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