Drivers could rake in millions of pounds as Sadiq Khan's Transport for London "broke the law" by using CCTV footage to fine motorists.
A panel of lawyers ruled against the capital's transport authority in a move which could result in penalty charge notices being refunded.
TfL broke guidance by using CCTV rather than civil enforcement officers (CEOs) to fine motorists parked in bays on red routes in London.
The Government rolled back the use of CCTV in 2015 after local authorities were found to have been "overzealous".
Sadiq KhanPAThe judgement on a total of eight appeals comes as Rishi Sunak vows to defend motorists.
The Prime Minister pivoted to backing drivers after the Tory Party held on to Uxbridge & South Ruislip on the back of anti-Ulez voters.
London Tribunals' ruling could enable further appeals across the rest of the country.
The Telegraph has claimed that around 435,000 PCNs of all kinds are issued by TfL each year.
Cars drive round the perimeter road at Heathrow AirportPAThe full charge of TfL fines is around £160, totalling £69million.
The adjudicators said: "A motorist parked in such a bay who encounters a civil enforcement officer may, there and then, be able to show that he or she is loading or unloading … or can readily obtain the evidence … to substantiate that claim."
Anthony Chan, the chief adjudicator who heard that case, added: “I am of the opinion that if a public authority becomes aware that the imposition of a penalty may be unlawful, it should seek to clarify the position before issuing more PCNs.
“Mr Korner does not think that is an issue because motorists can always challenge the PCNs … I do not find this argument attractive.”
Person driving carPAThree lawyers on the panel alleged TfL demonstrated “procedural impropriety” by issuing fines through CCTV.
A TfL spokesman said: “We are committed to keeping London moving safely and efficiently, and compliance on the Transport for London Road Network is essential to achieving these aims.
“Enforcement by our compliance officers using CCTV cameras is an important part of tackling this and we are exploring next steps.
"Non-compliance impacts London’s air quality, creates safety risks, disrupts traffic and creates congestion for everyone.”
