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's 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to accusations surrounding grants given to Ms Arcuri and trips she was taken on, which his solicitors have asked not to be published.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said: "We did finally receive a response from Boris Johnson, through his solicitors, which they have indicated may not be published.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.
"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."
There will be further responses in coming days, he added, while there are also other investigations underway.
The Independent Office for Police Conduct is looking into whether it should investigate if Mr Johnson committed misconduct in public office.
Ms Arcuri has said Mr Johnson was a "really good friend" of hers, in an ITV interview.
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