Scotland Yard has referred itself to the police watchdog over its refusal to launch an investigation into No10 parties last Christmas.
The Independent Office for Police Conduct is now assessing the issue and could decide today whether to launch a formal investigation.
IOPC head of national operations Sunny Bhalla had earlier told Green Party peer Jenny Jones, who made a complaint, the watchdog was “aware of the significant public anger” about the Mirror’s revelations.
The IOPC told her: “It is understandable that the events in question, which appear to have taken place whilst tight restrictions were imposed on the general public, are the subject of intense public and political scrutiny and debate.
“However, this does not alter the way in which complaints relating to these matters are handled.”
Separately, the Mayor's Office for Policing And Crime will also examine Baroness Jones’ complaint that Met Police Commissioner Cressida Dick "refused" to investigate the December 18 No10 Christmas party.
The IOPC initially wrote to Baroness Jones on December 8, saying her complaint had to be assessed internally by the Met Police before being referred to the IOPC.
The Met did so and then did refer the case to the IOPC on December 17. The watchdog is now assessing whether to launch a formal investigation, with a decision due as soon as today.
Baroness Jones had said there was a "case to answer" about whether the Met was "aiding and abetting a criminal offence, or deliberately failing to enforce the law in favour of Government politicians and their staff”.
She said the "extensive" police presence in Downing Street should have meant evidence was available for an investigation.
Acting Detective Chief Superintendent Tony O'Sullivan, of the Met’s directorate of professional standards, told Baroness Jones in a letter: "I have referred your complaint to the Independent Office for Police Conduct given that you effectively allege misconduct in public office by MPS police officers.
"The IOPC will now make a determination as to whether the complaint needs to be investigated and if so, how."
More than half a dozen parties last year in Downing Street and elsewhere in Whitehall have been revealed by media outlets including the Mirror.
We reported that around 40 staff were crammed in “cheek by jowl” for a No10 Christmas do on December 18, when London was under strict tier lockdown rules that banned indoor socialising.
Mr Johnson's former spokeswoman Allegra Stratton quit after being filmed joking about it with fellow aides at a mock press conference.
We also revealed a photo of Boris Johnson hosting a virtual No10 Christmas quiz on December 15, with two festively-clad staff members physically by his side.
And the Guardian has since published a photo of more than a dozen No10 staff - plus Mr Johnson and wife Carrie - having wine and cheese in the garden of Downing Street in May 2020, when gatherings in private gardens were illegal.
Officers have said they would speak to two people who were at a separate party in Tory HQ last December. But last week Scotland Yard insisted there was not yet enough evidence to launch a formal investigation into No10 bashes.
A spokesperson said on December 17: “The Met has received a significant amount of material in relation to the allegations reported in the media.
“All the material has been considered by detectives in detail and it does not provide evidence of a breach of the Health Protection Regulations, but restates allegations made in the media.
"In line with our policy where we do not normally investigate breaches of these regulations when they are reported long after they are said to have taken place, unless there is evidence from the Cabinet Office or other evidence comes to light, the Met will not at this time commence an investigation."
An IOPC spokesperson told the PA news agency: "We can confirm that, on Friday December 17, we received a referral from the Metropolitan Police Service of a complaint about an alleged party at Downing Street in December 2020.
"We are assessing it to determine what, if any, further action may be required from us.”
A spokesperson for the Mayor's Office for Policing and Crime said: "A complaint has been received and is under consideration."
A separate investigation is being led by senior civil servant Sue Gray into four parties across Whitehall. Its original boss Simon Case recused himself after it emerged a party was held in his own office.
