Motorists have unfairly paid Lord Khan's ultra-low emission zone (Ulez) charge for years despite driving compliant cars, The Telegraph can reveal.
Drivers of some imported vehicles may have been hit with fines and the £12.50 daily charge because of officials' refusal to accept foreign standards measuring how polluting engines are.
Cars imported from Japan are thought to be particularly affected. Earlier this year, Nissan was forced to write to Transport for London (TfL) with a list of vehicles from the early 2000s believed to have been caught out by the discrepancy, despite meeting emissions standards.
Ulez's registration system only recognises cars with European emissions ratings, and TfL will only accept vehicles manufactured overseas as compliant after extensive checks and documentation from the manufacturer.
Internal documents show TfL accepted Nissan's evidence that the vehicles – including the popular Nissan Cube and Fairlady Z – were likely compliant with the ultra-low emission zone, but decided not to exempt the cars and ruled out giving any refunds to drivers.
It means drivers must still go through the lengthy process of obtaining a Certificate of Conformity, proving their vehicle's compliance with Ulez, before they are let off the charges.
Sir James Cleverly, who is running against Lord Khan as the Tory candidate for Mayor of London, said: "TfL should refund anyone who was wrongly charged, and make it quick and simple for owners of these cars to be registered as compliant, without months of paperwork.
"Refusing to do either proves that Sadiq Khan cares more about squeezing drivers than about air quality," he added. "It's one more reason why, if I'm elected, I will review every charge City Hall imposes on Londoners. Too many of them hit the people who can least afford it, and Londoners deserve a system that is fair and transparent."
Used cars from Japan are popular in Britain because of their reputation for reliability and the fact they are right-hand drive, with UK importers reporting a booming trade.
It comes more than three years after the Ulez was extended by Lord Khan to cover all of Greater London. The controversial policy went ahead despite concerns that exemptions for vulnerable drivers were not generous enough.
As of April last year, the Mayor had collected around £394m in revenue from London's drivers through the scheme, according to figures from the Greater London Authority.
Multiple studies have supported the health benefits of the scheme, especially for young people, with one report by Queen Mary University of London claiming Ulez was linked to better lung development in children .
However, Lord Khan has also been accused of misleading the public over Ulez's benefits as well as trying to silence scientists who questioned its merits.
The landmark report used to justify the scheme's expansion in 2023 was signed off by Gary Fuller, an academic who was a member of a group that received funding from the Mayor's office.
Lord Khan's spokesman said it was "nonsense" to suggest Mr Fuller was anything other than an independent adviser and that it was "right and proper" that the "world leading academic" played a role in evaluating the impact of Ulez.
In April, new guidance was issued to Capita, which enforces Ulez on behalf of TfL, stating that imported Nissan vehicles manufactured since the 2000 financial year "will be accepted as compliant".
It caveats a general rule that only petrol cars registered from January 2006 – when the less polluting Euro 4 rating for new cars became mandatory – are exempt from paying the Ulez fee. Many Japanese engines conformed to this standard years earlier.
"This information has been gained by TfL working directly with Nissan and has only recently been confirmed by the manufacturer," the guidance goes on, before concluding that drivers of Japanese cars who were previously refused a Ulez exemption could not claim refunds.
A TfL spokesman said: "Vehicles built for markets outside Europe are subject to different emissions standards and regulations from those needed to comply with Ulez standards. In order for drivers to demonstrate that an imported car is compliant, they need to provide specific evidence from the vehicle manufacturer.
He added: "We have been working with Nissan UK to help it support its customers to submit the correct documentation that proves their vehicle is in line with the correct Euro emission standards. It is a long-standing principle of TfL's road user charging schemes that compliance can only be proved from the time that evidence is supplied."
