The London Assembly has asked Boris Johnson to explain by 5pm on Monday why his response to an investigation into his links to businesswoman Jennifer Arcuri should be kept confidential.
The prime minister wrote to the Assembly regarding the inquiry, insisting the contents of his response should remain private.
In the letter to Johnson’s solicitors, Len Duvall, chairman of the Assembly’s oversight committee, said: “Transparency and accountability are at the heart of the London Assembly’s operations and I was disappointed to see that your response is marked ‘Private and Confidential’ and ‘Not for Publication’.
“The reason for confidentiality is not given and, on the face of it, is unclear to me.
“I would request that you provide reasons for that confidentiality by 5pm on Monday 14 October 2019, so that the GLA oversight committee can consider appropriate handling of the letter.”
The oversight committee has asked Mr Johnson for details and a timeline of all contact with Arcuri “including social, personal and professional” during his time as mayor of London.
Johnson, who has denied any wrongdoing, previously accused his “old friends” in the Assembly of “barking up the wrong tree” with their investigation.