Motorists may be entitled to refunds amounting to nearly €700,000 (£500,000) after Police Community Support Officers (PCSOs) issued parking tickets without proper authority.
West Yorkshire Police said a search of its current records system, which goes back to 2006, showed about 16,000 parking tickets might be affected, totalling just over €670,712.42 (£485,000) paid to the courts.
Other tickets may have been issued in similar circumstances for up to three years earlier.
Assistant Chief Constable Andy Battle said in a statement: "Police carried out a review of PCSO powers in March 2015, at which time we discovered that some PCSOs had not been correctly granted powers to issue non-endorsable police fixed penalty notices for parking offences.
"This was due to an anomaly in the way that powers can be allocated and meant in effect that some PCSOs were not empowered to give tickets issued to vehicles illegally parked.
"The issue affects staff who joined the police first as PCSOs and were later granted authority to issue tickets.
"It does not affect those in our current staff who first worked as traffic wardens and later transferred into the force to become PCSOs. They already carried over relevant powers from their previous role as traffic wardens."
The force is assessing the scale of the issue, and what it means to motorists.
Mr Battle said: "We would like to emphasise that no PCSOs knowingly acted beyond their powers and they believed, as did the force at that time, that they were acting correctly.
"We want to reassure the public that this issue was addressed as soon as it came to light.
"Clearly this is a very complex issue and we are working through a process with HM Courts & Tribunals Service to establish how to resolve matters regarding affected motorists.
"This will include whether drivers issued tickets by PCSOs who did not have appropriate powers will be entitled to refunds for tickets issued, and, if so, how to set up a process for that to take place."
Establishing the full facts and working out a potential refund process is likely to take several weeks, Mr Battle added.