Parking fines totalling £485,000 could be refunded to thousands of motorists after a police force admitted errors in issuing the notices.
West Yorkshire Police said 16,000 tickets issued by some police community support officers (PCSO) were not valid.
PCSOs who joined the force several years ago were not initially granted powers to issue parking tickets.
West Yorkshire Police said it was working to resolve the issue of wrongly-issued tickets.
The force said it carried out a review of PCSO powers in March 2015 and "discovered that some PCSOs had not been correctly granted powers to issue non-endorsable Police Fixed Penalty Notices for parking offences".
Not empowered
In a statement West Yorkshire Police said: "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."
A spokesman said the problems did not affect current PCSOs who first worked as traffic wardens.
West Yorkshire Police said a search of the current records system, which goes back to 2006, "shows about 16,000 parking tickets may be affected, totalling just over £485,000 paid to the courts".
Other tickets may have been issued in similar circumstances for up to three years earlier, the force said.
It said it was working with the government's Courts and Tribunals Service on how to settle the matter.
"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," it said.
A spokesman for the West Yorkshire Police and Crime Commissioner Mark Burns-Williamson said: "At the time (in March 2015) the commissioner asked that the issue be addressed and the police have now done so".
http://www.bbc.co.uk/news/uk-england-leeds-34014848