Almost 350,000 parking fines totalling £23m may have been unlawfully issued to motorists in London.
In 2010 a ticket issued in a suspended parking bay was ruled unlawful because Camden Council did not have authorisation for its signage.
Now a BBC investigation has learned 16 councils still have no authorisation for these signs, while others went years without.
A typical inner London council suspends more than 1,500 parking bays a month.
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.
Otherwise they would be effectively licensed to invent road signs at will.
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.
And the DfT told the BBC’s Inside Out programme that another 16 councils still have no authorisation.
Neil Davies, a motoring solicitor at Caddick Davies, said: “From a legal perspective councils are on very shaky ground, because the signage they used is effectively made up.
“It’s difficult to explain the actions of councils who haven’t sought authorisation – they may be relying on the fact many people don’t challenge parking notices.”
The BBC has traced a minimum of 343,956 tickets issued under unauthorised signs.
Local Transport Minister Norman Baker said: “The department provides clear guidelines to councils to help them produce signs that comply with regulations.
Several authorities claimed a subsequent test case, a 2011 Court of Appeal judgement, meant their signage was nonetheless lawful.
That case established the legal principle that trivial failures to adhere to signage laws are not grounds to cancel a ticket if the sign is clear.
A London Councils spokeswoman said: “The Campbell case pre-dates an important decision in the Court of Appeal last year, where the court ruled a technical failure to comply with Traffic Signs Regulations does not invalidate signage so long as signs are clear and motorists are not misled.
“This ruling has effectively prevented further successful appeals on the grounds of a technical failure to comply with the regulations where no harm can be shown.”