The Greater Manchester Police has admitted to the loss of a USB stick containing data relating to an investigation.
The removable drive was stolen from an officer's home and the device is not thought to be password protected, the BBC reported.
That officer has now been suspended as the police carry out an investigation into the incident, whilst the Independent Police Complaints Commission and the Information Commissioner's Office (ICO) have been informed.
Just a single member of the public has been told their personal data was stolen.
"I want to reassure residents across Greater Manchester that we are taking this incredibly seriously and we are working hard to both identify who was responsible for the burglary and recover the stolen memory stick," said Assistant Chief Constable Garry Shewan.
"I want to stress that we do not believe the officer's home was deliberately targeted, but that this was an opportunistic burglary. I also want to make it clear that any suggestion this memory stick contained lists of names of people who 'tipped off' police is inaccurate."
An ICO spokesperson said the watchdog had been informed of a possible data breach, which "may involve Greater Manchester Police."
"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 spokesperson said.
A recent Big Brother report claimed police database abuse was "hugely intrusive."
Earlier this month, the ICO found the Lancashire Police force had breached the Data Protection Act.
