Almost two months on, Boris Johnson finds himself at the centre of a criminal investigation after Dame Cressida Dick, the Metropolitan Police commissioner, decided she could no longer overlook the ever-growing mound of allegations that shows no sign of abating.

The Telegraph understands that police are looking into eight out of 17 “parties” that have emerged since November, which took place over the course of 11 months from May 2020 to April 2021.

News of the police investigation prompted panic in Downing Street, where senior staff are well aware that if the Prime Minister is fined for breaking lockdown rules, he will surely have to resign.

At first it was seen as a brief stay of execution, as officials assumed the police investigation would delay the publication of the Cabinet Office report into alleged lockdown parties being carried out by Sue Gray, the senior civil servant.

By lunchtime that position had changed, with officials telling journalists that Ms Gray would be able to publish, at her own convenience, parts of her report that are not relevant to the police investigation.

Behind the scenes, that appeared to trigger a three-way row between No 10, the Cabinet Office and the Metropolitan Police after Dame Cressida made it clear that she had no objection to Ms Gray publishing her report in full this week. Downing Street was forced to fend off accusations that it was trying to block early publication of the report.

For weeks, police had been reluctant to get dragged into a high stakes investigation involving the Prime Minister.

But the turning point came on Sunday, when the Metropolitan Police received an outline of Ms Gray’s initial findings.

On the same day, a source revealed to The Telegraph that Ms Gray had spoken to Metropolitan Police officers stationed at Downing Street, who had given “damning” witness statements about events that happened there during lockdown.

After a team of officers assessed the information sent to them by Ms Gray, Dame Cressida was consulted and a decision was taken to launch a criminal investigation into a handful of the alleged parties.

Scotland Yard held back from announcing the investigation, however, because officers were expecting to receive a further update from the Cabinet Office on Monday, which included far more material.

Once that information was passed to them, detectives concluded that the inquiry would need to be widened to include more of the alleged events, the final tally being eight out of 17.

The decision on the scope and the scale of the police investigation was not finalised until around 8am on Tuesday morning, just two hours before Dame Cressida was due to make one of her regular appearances before the London Assembly Police and Crime panel.

Unmesh Desai, Labour’s policing and crime spokesman on the London Assembly, who was unaware of the Metropolitan Police’s change in position, had persuaded the chairman of the committee to allow an urgent question on “partygate”.

A source said he and other members of the committee were fully expecting to be “fobbed off” with an “unable to comment at this stage” response.

Instead, Dame Cressida dropped the bombshell that her officers were now investigating “potential breaches of Covid-19 regulations” over the previous two years in Downing Street and Whitehall.