The council's cameras were not certified until May 2007
More than 5,000 fines handed to motorists for driving in bus lanes in Greater Manchester are invalid.
The city council has admitted that it did not have the proper certification for some of its cameras beyond 24 December 2008.
It has been reviewing fines after more than 18,000 penalties paid between October 2006 and May 2007 were deemed invalid last year.
Drivers affected by the fines can contact the council about a refund.
The council said it understood from the Vehicle Certification Agency (VCA), which certifies the cameras, that it had the necessary approvals from 24 December 2008.
But the council now says "ambiguous phrasing" meant it did not realise it was not covered.
Vicky Rosin, the authority's director of neighbourhood services, said: "We are concerned and disappointed that a lack of clarity in the camera approval process has allowed this situation to arise.
"The council has acted in good faith throughout and taken steps to notify the public as soon as this came to our attention."
Ms Rosin said that all the current bus lane cameras have full approval.
The invalid fines affect some of the main routes in Greater Manchester including Oldham Road, Oxford Road, Bury New Road, Greater Ducie Street, Rochdale Road and Wilmslow Road.
The fines are invalid on different dates on the routes. A full list has been published on the council website.
Last year, the council faced a payout of more than £544,000 to drivers who were caught on cameras which were not certified.