The London Assembly has said it will investigate Boris Johnson's links to an American businesswoman with "absolute commitment", as it received a response from the prime minister an hour after the deadline.
The body - responsible for holding the Mayor of London to account - had requested that Mr Johnson provide it with evidence detailing his relationship with Jennifer Arcuri while he was in City Hall.
The London Assembly had given Mr Johnson 14 days to respond to questions over allegations Ms Acuri had received favourable treatment while he was London mayor.
But the prime minister replied on Tuesday evening - around an hour after the deadline.
- London Assembly Oversight Committee chair comments on Boris Johnson:
Despite his response, there was a spat between City Hall and those close to Mr Johnson over the deadline.
City Hall initially criticised him for missing the 6pm limit, with oversight committee chairman Len Duvall warning the PM the "already serious situation has now become grave".
A Tory source hit back saying it was missed because "it was an invented City Hall deadline that they have never said to us", and criticised London Assembly "grandstanding" and Mr Duvall for "playing politics".
The prime minister has been facing pressure over his links to Ms Arcuri, who allegedly received favourable treatment, including grants and access to business trips, thanks to her friendship with Mr Johnson at the time.
The assembly wrote to Mr Johnson, asking him to provide "details and a timeline of all contact with Jennifer Arcuri including social, personal and professional during his period of office as Mayor of London".
The letter also requested "an explanation of how that alleged personal relationship was disclosed and taken into account in any and all dealings with the GLA" (Greater London Authority).
Mr Duvall said: "Allegations of potential misuse of taxpayers’ money are no laughing matter. We will be taking this further."
Ms Arcuri has denied allegations of impropriety, saying she "never, ever" received favouritism from Mr Johnson while he was London Mayor.
But while appearing on ITV's Good Morning Britain, the technology entrepreneur and former model refused to answer when asked whether she had had a sexual relationship with Mr Johnson.
She said that it is "really categorically no-one's business what private life we had, or didn't have".