Downing Street officials hope the week can underscore their argument that Mr Johnson has got the “big calls” right in office and is focused on delivering for voters.

But with the Tories trailing Labour by double digits in opinion polls and large swathes of the public angered by the “partygate” allegations, the report will refocus attention on the saga.

Ms Gray, who has yet to comment in public about her investigation, is understood to be working within the Metropolitan Police’s limitations on what she can publish. Scotland Yard has told her to make only “minimal reference” to the events it is examining as part of its own “partygate” investigation.

That means the most controversial findings, at the events by which the police are most concerned, are not expected to be included in Ms Gray’s report. It is unclear whether the public will ever see what she found about the other events, which are now under police investigation.

The Telegraph understands that eight gatherings are being looked at by Met. Ms Gray has not announced when exactly she will give her findings to Mr Johnson.

Part of a statement released by Scotland Yard on Friday read: “In order to protect the integrity of the police investigation, as is appropriate in any case, and to be as fair as possible to those who are subject to it,  the Met has asked for minimal reference to be made in the Cabinet Office report to the relevant events.

“This will only be necessary until these matters are concluded, and is to give detectives the most reliable picture of what happened at these events. We intend to complete our investigations promptly, fairly and proportionately.

“We have not delayed this report and the timing of its release is a matter for the Cabinet Office inquiry team.”