The Greater London Assembly has given Boris Johnson until Monday to explain why his response to an inquiry into his links with Jennifer Arcuri should remain private.

The prime minister’s lawyers responded to the assembly’s request for information shortly after the deadline last week but made clear that his letter was “private and confidential” and “not for publication”.

However, the chair of the GLA’s Oversight Committee said the prime minister had “not provided the information requested” and asked why he wanted his response to be kept secret.

The committee is investigating allegations that Ms Arcuri, a technology entrepreneur, was given more than £100,000 of public money during Mr Johnson’s time as London mayor, as well as being invited on overseas trade missions that staff had deemed her unqualified for. Mr Johnson has refused to respond to claims about an affair between the pair, but has insisted that “everything was done with complete propriety”.

The GLA has the power to investigate anyone who has been Mayor of London within the last eight years.

It asked Mr Johnson to provide details of all his communications with Ms Arcuri during his eight years as Mayor between 2008 and 2016, along with an explanation of how his relationship with the former model was disclosed.

The prime minister now has four days to explain why his response to the request should not be made public.

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In a letter to the prime minister’s lawyers, Labour GLA member Len Duvall, who chairs 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London Assembly’s operations and I was disappointed to see that your response is marked ‘Private and Confidential’ and ‘Not for Publication’.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.”