Signs for suspended parking bays have never been authorised by the government for use in Waltham Forest, it has emerged - but the council insists fines are still valid.
An investigation by the BBC has revealed that Waltham Forest is one of 14 London boroughs which have failed to apply for permission to use their own designs for such signs.
The BBC says this could mean almost 350,000 parking fines in the capital - totalling an estimated £23million - could be invalid.
But Waltham Forest Council says only 48 fines in suspended bays have been issued in the borough over the last 10 years, and insists they are all legally valid.
A council spokesman dismissed the BBC's report as "a pretty weak case of investigative journalism".
However he added that the authority would now apply to get their signs authorised "in order to counter any further attempts to suggest wrongdoing".
The BBC's investigation is based on a court judgement in 2010 which overturned a fine in Camden on the grounds that proper permission for the signage had not been sought by the council there.
But London Councils, an umbrella body which represents local authorities in the capital, said a subsequent ruling meant this was not necessary.
Most road signs are designed and approved by the Department for Transport (DfT) before they can be used.
While parking bays are often suspended for building works, the DfT has never produced a version for suspended bays.
Legally councils must therefore seek permission to produce their own signs, but London Councils cites a 2011 judgement which said this was a "technicality" and that unauthorised signs were still valid as long as motorists were not misled.
But critics say council continued to apply for permission for signs since the ruling, suggesting such a position is not legally watertight.
London Councils said in a statement: “We have told the BBC that they have got this wrong in two ways.
"First, the Camden case does not provide a legal precedent.
"Second, in any case the law has since changed so that any clear sign is now allowed even if it does not formally comply with the signs regulations."
A council spokesman said that in Camden regulations explicitly stated only authorised signs could be used for temporary suspension of bays.
But he said in Waltham Forest traffic orders meant only 'no waiting' or 'no stopping' signs could be used with "additional information" alongside them which the council insists cannot be classed as part of a sign.
He added: "What this all adds up to is a pretty weak case of investigative journalism that leaves as many questions as answers in terms of the historic fines, but needless to say we are confident that no motorist issued with one of the 48 tickets was misled.
"They parked in suspended bays that were clearly and legally signed to that effect, were subsequently issued with tickets which they paid."
But Richard Bentley, a former police officer and sign consultant, told the BBC: "Each council is fully aware they have to apply to the secretary of state if they want to use signing that isn't set out within the regulations.
"It is astounding authorities ignore the very laws there to help them."