The Greater London Authority’s monitoring officer, Rory McKenna, has decided to appoint an independent investigator to look into a series of complaints received about the mayor from Nick Rogers, a Tory member of the London Assembly.
These relate to the consultation run by Transport for London last summer, which found two-thirds of respondents opposed the expansion.
Mr Rogers claimed that Mr Khan “knowingly made false and dishonest statements” to the London Assembly last October and November in relation to the consultation and his knowledge of the responses to it.
Sources told the Standard that the investigation would also look into a People’s Question Time public meeting in Ealing in March, during which Mr Khan said Ulez opponents were “holding hands” with the far-Right.
Mr Rogers, transport spokesman for the City Hall Conservatives, first made the complaint in January.
The Ulez expanded Londonwide on August 29 to widespread controversy, with more than 500 enforcement cameras having been vandalised or stolen.
At the Ealing meeting, a group of protesters were seen outside with signs that depicted the Mayor with a swastika and a hammer-and-sickle symbol.
During the meeting, Mr Khan responded to heckles from the audience by saying: “What I find unacceptable is some of those who’ve got legitimate objections [about Ulez] joining hands with some of those outside, who are part of a far-Right group.”
This prompted some audience members to shout back: “We are not the far-Right — normal people are not the far-Right.”
The complaint from Mr Rogers claimed that Mr Khan misled and made “false and dishonest” statements to the London Assembly over what he knew about responses to the Ulez consultation.
Mr Khan is typically subject to a number of complaints each year, mostly from political rivals. Some are thrown out at an early stage, while others progress to the investigation stage.
The decision to appoint an independent investigator does not mean there has been a presumption of wrongdoing.
Previous complaints against the mayor, such as in relation to the departure of former Met commissioner Dame Cressida Dick and the opening of the Elizabeth line, have not been upheld.
A spokesperson for Mr Khan said: “The mayor has always been clear that expanding the Ulez was a difficult but necessary decision, and he’s been listening closely to those with concerns. The mayor looks forward to the outcome of the monitoring officer’s investigation.”
Mr Rogers said: “I believe the evidence clearly shows that the Mayor of London, Sadiq Khan, made misleading statements to the Assembly and to Londoners over the Ulez expansion consultation; specifically that he told us repeatedly that he had no knowledge of how it was progressing.
“The City Hall monitoring officer has now taken forward the complaint I made against Sadiq Khan over his conduct during the implementation of his London-wide Ulez expansion.
“Londoners will feel vindicated that the complaint made against the mayor is being taken seriously and will be independently investigated.
“He should now do the right thing, listen to Londoners and cancel his Ulez expansion immediately.”
In his first mayoral term, Mr Khan was reprimanded by the monitoring officer for a press release which claimed there was appalling incompetence and infighting in Theresa May’s Government, and for discussing his re-election plans in an interview with Pink News that was done in his City Hall office.