Scotland Yard asked Sue Gray to make only “minimal reference” to events now subject to a criminal investigation, throwing her report into disarray and potentially buying more time for Boris Johnson as he faces a threat to his leadership.
Labour leader Sir Keir Starmer claimed the scandal had "paralysed" the Government, while Nicola Sturgeon claimed things were getting "murkier by the minute".
The force argued the constraints on the Cabinet Office report are necessary to “avoid any prejudice to our investigation”, meaning it faces being watered down or a lengthy delay.
In a statement on Friday morning, 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.”
Opposition politicians warned of a “stitch-up” amid growing calls for the official report into potential Covid breaches in Downing Street and wider Government to be published in full, with it having the potential to trigger a vote of no confidence in Mr Johnson.
Writing on Twitter, the First Minister warned any delay could “help” the Prime Minister.
She said: “This 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
“I doubt Johnson cares about damage to the reputations of others - individuals or institutions - as long as he saves his own skin. But these things matter. Rapid conclusion and full publication of the findings of inquiries surely now essential for public trust.”
Sir Keir called tor the full report to be published “as soon as possible” but conceding that “any issues of prejudice have got to be worked through”.
He said: “The more that anybody sees Boris Johnson, the more they’re frustrated at the fact that, through his misbehaviour, we now have a civil service investigation and a criminal investigation into our prime minister and into what went on in Downing Street.
“That the whole of Government is paralysed and not focused on the things that in Scotland most people here talking about: the cost of living, the price hikes, the energy price hikes, their bills are going up, they’re really worried about inflation and they know that the Government’s about to whack them with more tax – their focus is completely wrong.”
Ken Macdonald, the former director of public prosecutions (DPP), claimed the move seems “disproportionate” in the face of “very powerful” public interest in the report’s swift publication, unless there is “more serious conduct” being investigated.
He continued: “I very much doubt that anything that Sue Gray says is going to come as any great surprise to any of the protagonists so it’s all a bit of a mystery.
“Unless there is some other more serious conduct in play here that we’re not aware of, I do think the police approach this morning has been surprising and in many ways quite unhelpful.
“The risk of the police intervention this morning is that this leaves things hanging in the air for weeks and months and that seems obviously not to be in the public interest.”
Sources close to the inquiry previously indicated Ms Gray was concerned about the prospect of releasing a report that was shorn of some of its key findings, raising the likelihood of a significant delay.
Officers have not confirmed how many events they are investigating, but reports have suggested it could be as high as eight.
Conservative MP Sir Christopher Chope accused the Met of “usurping its position by seeking to interfere in the affairs of state”.
Raising a point of order in the Commons, he said: “There is no reason for the Metropolitan Police to be able to require Sue Gray not to issue her report in an unamended way for the benefit of the Prime Minister who ordered that report, and for this House, which is eager to see that report.”
His colleague Sir Roger described the police intervention as “ridiculous”, saying: “Unless there is a legal barrier to Sue Gray publishing her report then I believe that it should be published now and in full.”
The spokeswoman for the Covid-19 Bereaved Families for Justice said: “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.”
Liberal Democrat leader Sir Ed Davey said: “A stitch-up between the Met leadership and No 10 will damage our politics for generations, and it looks like it is happening right in front of our eyes.”
SNP Westminster leader Ian Blackford called for the report to be published “in full and undoctored without further delay”.
“People are understandably concerned that this increasingly looks like a cover-up,” he added.
Theresa May said she was “angry” at the allegations of parties and that “nobody is above the law”.
It is believed the Met decision is related to officers’ ability to effectively investigate, rather than because more serious offences were being probed.
Officers were understood to be looking into possible breaches of Covid rules that may warrant fixed penalty notices.
So far seven Tory MPs have publicly called for Mr Johnson to quit, but others are believed to have done so privately in letters to the chairman of the Conservatives’ 1922 Committee.
If the number of letters received by Sir Graham Brady hits 54, representing 15 per cent of all Tory MPs, then a vote of no confidence in the Prime Minister’s leadership is triggered.
Mr Johnson would have to then win the support of half of Conservative MPs in order to stay in No 10.
It came as the minister who claimed Mr Johnson was “ambushed with a cake” at a birthday gathering during coronavirus restrictions has now said there was no cake and therefore the event was simply a meeting.
Northern Ireland minister Conor Burns defended the Prime Minister amid reports his wife Carrie had organised a surprise birthday get together on June 19, 2020, in the Cabinet room which 30 people attended.