Downing Street staff will be allowed to view Sue Gray's Partygate notes about them before they respond to the police.
Scotland Yard has sent formal questionnaires to more than 50 staff regarding lockdown-flouting gatherings in Downing Street and Whitehall during the pandemic.
It has now emerged that people under investigation - which includes Boris Johnson - will get limited access to their own interviews with Ms Gray's inquiry before they respond to the Metropolitan Police.
Mr Johnson has until the end of the day to submit his Partygate questionnaire to the Met after consulting his lawyers over his response.
In a leaked letter to staff, obtained by ITV, Ms Gray said that "in light of particular circumstances surrounding this set of events I have, as an exceptional measure, decided that individuals may be provided with limited access to the notes".
Ms Gray admitted this was not "standard practice" in internal inquiries but said it would allow staff to maintain consistent answers.
Top barrister Adam Wagner branded the decision "bizarre" and questioned why staff needed help to provide consistent answers to officers.
He tweeted: "Every member of staff should be saying what they remember about the events - the Gray interviews only took place a few weeks ago so it is not as if their memories would have been any better then than now. It's all very odd."
Liberal Democrat leader Ed Davey also questioned the decision which he warned could undermine public trust.
He said: "Decisions like this do nothing to improve public confidence. Any impression of conspiracy or cover-up would be disastrous for public trust in both government and the police."
No 10 staff will only be allowed to view notes about themselves and cannot challenge the content.
They are also banned from taking pictures or video of the documents.
Ms Gray handed over evidence she obtained in her Partygate inquiry to the Metropolitan Police - included 300 photos - after being forced to delay publishing her full findings until the police probe concluded.
Detectives are investigating 12 events for potential Covid rule breaches, including six gatherings the Prime Minister is believed to have attended.
These include a BYOB bash in the Downing Street garden in May 2020 and a party in his No 11 flat in November 2020.
Police are also reviewing their decision not to investigate a Downing Street Christmas quiz in December 2020 after the Mirror published a picture of the PM and staff alongside an open bottle of bubbly.
Ms Gray is looking at 16 events in total, which includes the 12 examined by cops.
Downing Street has said it will reveal if Mr Johnson is fined by police but the identities of other No 10 staff could be kept secret.
The Met is due to reveal how many fixed penalty notices are doled out but it will not name those involved.
Mr Johnson is expected to resist attempts to oust him over Partygate but many Tory MPs are waiting for the results of the police inquiry before they decide whether to press for a no confidence vote.
The Cabinet Office and Downing Street were contacted for comment.
