The next day, after weeks of pressure, the Met, which had stuck to the line that it did not retrospectively investigate historic breaches of Covid rules, finally caved in and opened Operation Hillman, its own investigation into “partygate”.

It said it was looking into 12 gatherings, including at least three attended by the Prime Minister.

The pivotal moment came when the Met received an outline of the initial findings by Ms Gray, who had spoken to police officers stationed at Downing Street and taken “damning” witness statements about events that happened there during lockdown.

Last week Helen MacNamara, the Government’s former director of propriety and ethics, became the first person to confirm she had been fined, after attending a leaving party in the Cabinet Office for a colleague in June 2020.

To date, the investigation has resulted in 50 fines being issued over “partygate”, but there will almost certainly be more.

The Prime Minister’s surprise birthday get-together was distilled into the cold language of a police report when he was informed by Scotland Yard that: “On 19th June 2020 at the Cabinet Room, 10 Downing Street, between 1400 and 1500 you participated in a gathering of two or more people indoors.”

Issuing a “full apology”, Mr Johnson said the gathering lasted for only nine minutes, adding: “I have to say in all frankness at that time it did not occur to me that this might have been a breach of the rules. I now humbly accept that it was.”

A spokesman for Mrs Johnson said she “apologises unreservedly”, although “she believed that she was acting in accordance with the rules at the time”.

As well as having broken the law, both Mr Johnson and Mr Sunak face accusations that they lied to Parliament, which Labour has framed as a resignation issue.