London's Metropolitan Police force said on Sunday it was taking security measures after "unauthorised access to the IT system of one of its suppliers".
The move follows recent major data breaches at other forces that came to light earlier this month, including at the Police Service of Northern Ireland.
The Met said the supplier company in question had access to "names, ranks, photos, vetting levels and pay numbers for officers and staff".
"The company did not hold personal information such as addresses, phone numbers or financial details," the Met added.
'Sense of fury'
"Metropolitan Police officers are - as we speak - out on the streets of London undertaking some of the most difficult and dangerous roles imaginable to catch criminals and keep the public safe," said Rick Prior, vice chairman of the Metropolitan Police Federation, which represents rank and file officers.
"To have their personal details potentially leaked out into the public domain in this manner - for all to possibly see - will cause colleagues incredible concern and anger.
"We share that sense of fury... this is a staggering security breach that should never have happened."
The Met has reported the case to the National Crime Agency.
The incident follows an admission by the Police Service of Northern Ireland that personal data on all its serving members was mistakenly published. The admission came in response to a Freedom of Information request.
Details of around 10,000 PSNI officers and staff included the surname and first initial of every employee, their rank or grade, where they are based and the unit they work in.
After that data breach came to light, Norfolk and Suffolk Police announced the personal data of more than 1,000 people, some victims of crimes, had been exposed.
In addition, on Wednesday South Yorkshire police referred itself to the Information Commissioner's Office, after noticing "a significant and unexplained reduction in data stored on its systems".
Some of that data was footage filmed by officers as they attended incidents or engaged with the public, which, in some cases, could be used as evidence in court.
