An east London council is refunding up to £347,000 to drivers after parking tickets were illegally issued by a dozen cameras.
Between 2009 and last year, Newham Council issued tickets to 4,952 people from unauthorised cameras.
Last June, it refused to refund affected drivers but it has now written to them offering a total of £347,000.
A Newham Council spokeswoman said: "Steps have been taken to ensure the certification is maintained in future."
It added that so far 2,643 refunds had been claimed, with 1,192 claims processed and £106,843 refunded.
Margaret Stroud, of B&P Carpets, which was illegally fined in May 2009 but not contacted to be offered a refund until this March, said: "It's taken a long time to get offered a refund. We feel quite upset and it has cost the business money to deal with this.
"We are frustrated with all of Newham's charges. It was an aggravation the business couldn't afford."
She added: "There might be lots of people who were illegally charged and do not know how to claim back. "
By law, only camera models that have been specifically authorised by the Vehicle Certification Agency (VCA) can be used by councils.
Otherwise authorities might use cameras that are not of a high enough calibre to be relied upon for evidence.
But Newham Council used cameras that did not have approval.
In June, the council cancelled all the parking tickets that were unpaid, writing off a total of £216,133, but it refused to refund those drivers who settled the unlawful fines straight away.
However, it has now written to 4,952 people who are eligible for a refund after being issued Penalty Charge Notices (PCNs) between March 2009 and February last year.
When the problem came to light in February last year, Newham Council commissioned accountancy firm PricewaterhouseCoopers to carry out an audit of how many unlawful tickets it had issued.
BBC London obtained a final copy of that report in June, which covers tickets issued in the years 2011-2012 and 2012-2013.
It read: "Compliance is high (97%); however, due to the volume of parking penalty charge notices issued even a small percentage of non-compliance will have a significant impact on the council's finances."
Steve Baker, of the NoToMob, which campaigns on behalf of motorists, said: "The NoToMob welcome the council's decision to return the money to those it has wronged.
"But we are most disappointed that Newham Council have had to be dragged, kicking and screaming to the point where they are finally going to do what's right."
A Newham Council spokeswoman said: "The PCNs eligible for a refund were issued between 31 March 2009 - 6 February 2013 by a small number of cameras that were not properly certified at 12 locations.
"The problem was identified and corrected. To date, 2,065 people have applied for a refund and only 134 of these are Newham residents."
Local government minister Brandon Lewis said: "This government is reining in over-zealous parking enforcement and unfair parking practices, with the levels of parking penalty charges being kept under review."