The Met Police says it has received new evidence from the Cabinet Office as part of its investigation into possible breaches of Covid regulations in Downing Street.
The force is looking at "a number of events" in Downing Street and Whitehall during the pandemic.
Scotland Yard has asked Sue Gray to make "minimal references" to the parties she is investigating in her report to avoid prejudicing its inquiry.
However, officers said they had not delayed the publication of the report.
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The force has not specified which events it is investigating but reports have suggested it could be as many as eight gatherings, according to The Mirror.
At the same time, Sue Gray's report had been expected this week but the timing has been thrown into chaos by the Met's bombshell announcement of a police probe on Tuesday.
The Whitehall inquiry is understood to be still ongoing but Ms Gray may be forced to decide whether to delay her report or strip out key details.
Opposition MPs blasted the appearance of an "establishment stitch-up" and a "Whitehall whitewash", which could buy Boris Johnson a reprieve.
The Prime Minister will spend the weekend at his Chequers retreat as he seeks to shore up his battered leadership.
Commander Catherine Roper, who leads the Met’s Central Specialist Crime Command, said officers would examine the new evidence 'to establish whether individuals attending the events in question may have breached the regulations'.
She added: "They will do so without fear or favour following our normal processes.
“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."
However some legal figures questioned why publishing the Whitehall report would prejudice the police investigation.
Nazir Afzal, a former chief Crown prosecutor for the North West, said on Twitter : "This is absolute nonsense from the Met Police. A purely factual report by Sue Gray cannot possibly prejudice a police investigation.
"They just have to follow the evidence, of which the report will be a part."
Top barrister Adam Wagner, who is an expert on Covid rules, tweeted: "I am not a criminal lawyer so perhaps I am missing something.
"How would a factual civil service report about events the police is investigating 'prejudice' their investigation?"
