London's Metropolitan Police force said Sunday it was taking security measures after "unauthorised access to the IT system of one of its suppliers", following data breaches at other forces.
The company in question had access to the names, ranks, photos, vetting levels and pay numbers for officers and staff, but not addresses, phone numbers or financial details, it said.
The Sun on Sunday newspaper reported that "cyber crooks penetrated the IT systems" of the firm which reportedly prints identity cards and staff passes for the Met, the UK's biggest police force.
Scotland Yard said the force was now working with the company to understand if there had been any security breach relating to its data.
According to a spokesman, it was unable to say when the breach occurred or how many personnel might be affected.
"Security measures have been taken... as a result of this report," the force said in a statement.
The Metropolitan Police Federation, which represents rank and file officers, said the breach would "cause colleagues incredible concern and anger".
"We share that sense of fury… this is a staggering security breach that should never have happened," said vice chair Rick Prior.
It follows an admission this month by the Police Service of Northern Ireland (PSNI) that personal data on all serving members was mistakenly published in response to a Freedom of Information (FOI) 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 were based and the unit they worked in.
The error comes months after the terrorism threat level in the UK-run province was increased to "severe" in response to an assassination attempt on a senior police officer by dissident republicans.
After the PSNI breach was revealed, Norfolk and Suffolk Police also announced that the personal data of more than 1,000 people –- including crime victims –- was included in another FOI response.
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".