Lawyers have questioned how Sue Gray's report could prejudice a police inquiry as fears mount that the Downing Street partygate probe could be delayed or watered down.
The Metropolitan Police asked the top civil servant to make "minimal references" to the parties it is investigating in her report to avoid prejudicing its inquiry.
Scotland Yard is looking at "a number of events" in Downing Street and Whitehall during the pandemic - but the force has not specified which ones.
Reports have suggested it could be as many as eight gatherings.
Ms 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.
In a statement on Friday, Scotland Yard said: "For the events the Met is investigating, we asked for minimal reference to be made in the Cabinet Office report.
"The Met did not ask for any limitations on other events in the report, or for the report to be delayed, but we have had ongoing contact with the Cabinet Office, including on the content of the report, to avoid any prejudice to our investigation."
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?"
Breaches of lockdown rules are summary offences, which result in a fine and very unlikely to result in prosecution.
Mr Wagner suggested that police could be worried about the possibility of a future jury trial over an offence like misconduct in public office.
There is some speculation that the Sue Gray report could open the door to possible action for perverting the course of justice, if officers discover those involved lied or tried to conceal evidence.
But Mr Wagner added: "It still seems odd to say that Sue Gray's findings would at this very early stage "prejudice" anything."
Nick Aldworth, a former Metropolitan Police chief superintendent and counter-terrorism national co-ordinator, said the report could prejudice the police investigation "by disclosing the evidence that they will gather and thereby giving the potential defendants an opportunity to conceal or alter evidence".
Barrister Andrew Keogh tweeted: "'Criminal proceedings" does not just mean a trial, so prejudice to them also means the investigation.
"Giving a potential suspect early sight of other evidence may for eg risk prejudice. We need to think wider on this one. Police slammed for not acting, now for doing it properly."
Former Tory PM Theresa May said this week that she was "angry" at the allegations of parties in Downing Street - and warned "no one is above the law".
According to the Maidenhead Advertiser, her local paper, Ms May said: "I have said previously that it is vital that those who set the rules, follow the rules. Nobody is above the law.
"This is important for ensuring the necessary degree of trust between the public and Government.
"Like so many, I was angry to hear stories of those in No 10, who are responsible for setting the coronavirus rules, not properly following the rules."
Keir Starmer said the Government had been "paralysed" by the Sue Gray investigation and said he wanted to "see Sue Gray's report in full and the investigation finished as quickly as possible".
The Labour leader said: "We've got a criminal investigation into the behaviour of the Prime Minister and what went on in Downing Street.
"There are bound to be process issues along the way, but this is caused by one thing, and that's the behaviour of the Prime Minister."
Asked whether, with his legal background, he saw any issues of prejudice, Sir Keir said: "Any issues of prejudice have got to be worked through but this whole mess, this whole of paralysing of politics, is being caused by the Prime Minister and his wrongdoing."
Scottish First Minister Nicola Sturgeon said the saga "gets murkier by the minute".
"Sue Gray and the Met are in difficult positions but the sequence of events and the situation arrived at now creates the suspicion - however unfairly - that the process of inquiry is aiding Johnson at the expense of public accountability," she tweeted.
The Liberal Democrats warned that the appearance of an "establishment stitch-up" would be "profoundly damaging" to public confidence in the police.
Home Affairs Spokesperson Alistair Carmichael said: "Police officers need the trust and confidence of the public to do their jobs and keep our communities safe.
"That's why we called for the police to investigate No 10 weeks ago and put this whole sorry business behind us, instead of waiting for Sue Gray.
"The Sue Gray report must be published in full, including all photos, text messages and other evidence. If it is redacted now, a full, unredacted version must be published as soon as the police investigation is complete."
SNP Westminster Leader Ian Blackford said the full report needed to be published.
He said: "This UK Government farce has gone on long enough. People are understandably concerned that this increasingly looks like a cover up.
"It is a fact that Boris Johnson broke the rules, lied about it, and misled Parliament. This is an extremely serious resignation matter - and he must be held to account.
"The Prime Minister cannot be allowed to wriggle off the hook by using the Metropolitan Police investigation as an excuse to further delay or doctor the report.
"This cannot be another Whitewall whitewash."
Fran Hall, whose husband was a police officer for more than 30 years before he died of Covid in 2020, said the Gray inquiry had becomes "a circus".
Ms Hall, a spokeswoman for the Covid-19 Bereaved Families for Justice said: "Tragically, it seems here that the Metropolitan Police have broken the trust of the public by first refusing to investigate flagrant law-breaking, and now demanding any other investigations hide the most serious illegalities happening at Downing Street.
"It's incredibly painful and they have let families like mine down. My husband was completely committed to justice, and he would have been appalled by this."
