The Metropolitan police is understood to have advised that the next People’s Question Time (PQT) event should not go ahead in Westminster in November as planned.
This follows ugly scenes at a PQT in March, when it was held at Ealing town hall. Anti-Ulez protesters, some of whom Mr Khan described as being far-Right, gathered outside the meeting and there was a hostile atmosphere inside.
Mr Khan is currently facing a City Hall standards investigation for saying at the Ealing meeting that Ulez opponents were “holding hands” with the far-Right.
Two PQT events a year have to be held by law. They are held in different boroughs to enable as wide a selection of Londoners as possible to attend at least once during each mayoral term.
The Met has raised concerns about the Mayor’s attendance at a major event being publicised in advance, and has suggested the November PQT be held online or at City Hall in east London.
But this has been rejected by senior figures on the London Assembly, who say it is essential for Londoners to be able to hold the mayor to account in public.
Tony Devenish, a Tory member of the London Assembly, said: “Are the Met police really saying they can’t protect the Mayor of London when he is in the City of Westminster?
“We all understand if there is a spike in security concerns or a specific threat, but a move like this just damages democracy.”
Some assembly members believe that an overstretched Met may be seeking to limit attendance at public events to enable it to focus on frontline crime.
Met commissioner Sir Mark Rowley recently appeared to suggest that he would appear less frequently before the assembly to answer questions because he would be required to attend the mayor’s new policing board, which meets for the first time next Tuesday.
A mayoral spokesman confirmed that security concerns had been raised but said the event would be held in public, though possibly outside Westminster.
Mr Khan occasionally travels in an armed police vehicle when travelling around London, on police advice, but tries to use the Tube when possible.
In February, a Ulez protester was led away from a City Hall event involving Mr Khan and the moneysaving expert Martin Lewis.
A book launch event at the South Bank in May was also marred by anti-Ulez protests, with a number of people escorted out of the building by security staff after shouting profanities at the mayor.
The Met police has been approached for comment.