Almost 350,000 parking fines - totalling an estimated £23 million - may have been unlawfully issued to motorists in London, a BBC report suggests.
In 2010, a ticket issued in a suspended parking bay was ruled unlawful because Camden Council did not have authorisation for its signage.
Now the BBC says it has learned 16 councils still have no authorisation for these signs, while others went years without.
Some boroughs insist a later judgement made tickets enforceable.
A typical inner London council suspends more than 1,500 parking bays a month often so building works can take place.
The Department for Transport (DfT) designs road signs for most situations, which authorities must follow closely, but it has never produced a template for a suspended parking bay sign.
If no sign is set out by the DfT, the law says councils must ask the transport secretary to authorise their own creations.
The following councils still have no DfT authorisation for their signs: Greenwich, Southwark, Westminster, Barnet, Bexley, Bromley, Croydon, Ealing, Enfield, Hillingdon, Kingston-upon-Thames, Merton, Redbridge, Sutton and Waltham Forest.
The following councils received authorisation in 2010 or after: Camden, Hackney, Lambeth, Harrow, Wandsworth, Havering, Barking and Dagenham, Brent, Newham, Hounslow, Lewisham and Haringey.
At the time of research Richmond-upon-Thames was applying for authorisation but did not yet have it.
In January 2010, motorist Suzanne Campbell defeated Camden Council at a Parking and Traffic Appeals Service (Patas) hearing after being ticketed in a suspended parking bay.
Adjudicator Edward Houghton ruled: "In the absence of a compliant sign the vehicle was not in contravention and the appeal must be allowed.
"No doubt the council will give consideration to obtaining the secretary of state's authorisation."
Shortly afterwards there was a rush of applications for authorisation from London councils. Some 14 received it by 2012.
But all these councils had been issuing tickets in suspended parking bays for years previously.
According to the DfT, another 16 councils still have no authorisation.
The BBC sent Freedom of Information requests to all 28 councils which had no authorisation prior to 2012 to find out how many potentially-unlawful tickets were issued.
Some councils provided a decade's statistics, others just two years.
But the BBC has traced a minimum of 343,956 tickets issued under unauthorised signs. The real number is probably far higher.
According to a Westminster City Council report, London boroughs make £67 per parking ticket - a total revenue of more than £23 million.
However, motorists fined years ago might find it hard to claw money back - as there is a time limit of 28 days to make an appeal.
To find out more, watch the London edition of BBC Inside Out, on BBC on the iPlayer.