According to the Sunday Telegraph, a draft report by the Advertising Standards Authority (ASA) has found that TfL - which is chaired by Mr Khan - produced ads which “were likely to mislead” viewers.
This is because they “did not clarify” that claims about air pollution levels were based on estimates, rather than actual figures.
Asked about the matter by the Standard this week, the mayor said: “I don’t know a lot about it.”
“What I do know is there’s a disagreement about wording, to do with an advert that’s I think almost a year old.
“I think TfL are speaking to the ASA. I think it’s a draft report, from what I’ve seen in the media, and hopefully TfL and the ASA will resolve it.”
Pressed on whether it was an issue that concerned him, Mr Khan said: “No, the science is sound. It’s just a question of a disagreement about wording.”
The adverts, which were broadcast and published between January and June, claimed that the existing Ulez in central London had “almost halved levels of nitrogen dioxide (NO2)” and that “most air pollution related deaths actually occur in outer London”.
The draft of the watchdog’s conclusions found that the adverts were misleading because they “did not clarify” that claims about NO2 levels were based on estimates or modelled scenarios and not actual figures.
A TfL spokesman told the Sunday Telegraph it was “strongly challenging” the draft recommendations.
“The ASA is not challenging the science... the science is absolutely clear about the significant harm of air pollution on people’s health and that estimated premature deaths from air pollution are higher in outer London than in inner London. We are confident that the advertisement is accurate,” he said.
“Scientific analysis based on modelled scenarios and estimates is standard practice in the scientific community. We are meeting with the ASA to take them through the data and explain in detail how it is used.
“It remains the case that the expansion of the ultra low emission zone is playing a crucial role in the reduction of air pollution - improving air quality for everyone in London and reducing the harms to health associated with vehicle emissions.”
The ASA refused to comment to the Sunday Telegraph on the leaked document but a source told the newspaper that the final ruling had not been issued and so the findings could change. The final ruling is set to be officially released in the next few weeks.
A total of 578 people were said to have contacted the ASA to “challenge” TfL to prove its adverts’ claims.