Boris Johnson's response to allegations surrounding his relationship with US businesswoman Jennifer Arcuri when he was London Mayor were "insufficient", according to the London Assembly.
The PM sent correspondence in regards to accusations surrounding grants given to Ms Arcuri and trips she was taken on, which his solicitors have asked not to be published.
The London Assembly said it did not see a reason for them to be confidential though it is respecting the request, after receiving the response after its initial deadline yesterday afternoon.
Next steps are being considered which might include the power to summons the Prime Minister to give evidence on the alleged conflict of interest.
Len Duvall AM, chair of the oversight committee, said: "We did finally receive a response from Boris Johnson, through his solicitors, which they have indicated may not be published. At this stage we are respecting that, but we are seeking further clarification.
"Nothing in the response, in our opinion, reflects the need for confidentiality. In fact, the response is insufficient as far as our request for information is concerned."
Labour's Mr Duvall said his committee was considering using "our power of summons" in order to hear Mr Johnson's testimony in public, and confirmed a decision on what action they would take would be "finalised in the coming days".
He added: "We are focused on our investigation and considering next steps. A number of options are open to us; they include speaking to various people and using our power of summons."
The Assembly had asked Mr Johnson to provide details and a timeline of all contact with Ms Arcuri while he was the mayor.
It also asked him to give an explanation of how that relationship was disclosed and taken into account in all dealings with the assembly and related bodies.
Read More
The Independent Office for Police Conduct is looking into whether it should investigate if Mr Johnson committed misconduct in public office.
Ms Arcuri used an interview on Monday with ITV's Good Morning Britain to deny reports that she received favouritism during his eight-year stint as mayor.
She described Mr Johnson as a "really good friend".
Mr Johnson has previously accused his "old friends" in the Assembly of "barking up the wrong tree" with their investigation.
Despite the PM's response on Tuesday, there was a spat between the London Assembly and those close to Mr Johnson over the deadline.
City Hall initially criticised him for missing the 6pm limit, with Mr Duvall warning the PM the situation had "now become grave".
A Tory source hit back saying 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 PM's response was received about an hour after the deadline.
Those who fail to respond to Assembly summons for evidence can face up to three months in jail.