Boris Johnson has warned City Hall it may be "overstepping the mark" by asking for intimate details about his relationship with Jennifer Arcuri.
A letter from the Prime Minister’s lawyers to the head of the London Assembly oversight committee claims that it is not in “the public interest” for an investigation to take place as it “risked becoming politicised”.
The letter, sent by solicitors Atkins Thomson, fails to contain the requested details of "personal, social and professional" contact between the former London mayor and Ms Arcuri.
Committee chairman, Len Duvall, had previously given Mr Johnson a deadline of Wednesday evening to send over the information, but instead received a letter from lawyers questioning his right to investigate.
It was marked ‘private and confidential’ but has now been leaked.
Mr Duvall said previously: "We did finally receive a response from Boris Johnson, through his solicitors, which they have indicated may not be published.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."
Since receiving the letter, Mr Duvall has asked for legal advice about whether the committee can discuss its contents at an open meeting next Wednesday.
The meeting is also expected to decide whether the committee should use its powers to summon the Mr Johnson to answer questions in person.
The Prime Minister is facing accusations of failing to declare a conflict of interest over his relationship with Ms Arcuri, after the American businesswoman received more than £100,000 in public funding and privileged access to three foreign trade missions.
Ms Arcuri used a TV interview this week to deny reports that she had received favouritism during Mr Johnson's eight-year term as mayor.
However, she did admit that he had been a “really good friend” and repeatedly refused to deny their relationship had been sexual.
Mr Johnson has also failed to answer questions on the nature of his relationship with the former model, but previously stated "everything was done with complete propriety".
The Independent Office for Police Conduct is currently deciding whether it should investigate if Mr Johnson committed misconduct in public office, after a formal referral from the Greater London Authority.