It follows multiple reports of lockdown-busting gatherings allegedly taking place in No10 and government departments at a time when the country was under coronavirus restrictions.
Green Party peer Baroness Jenny Jones wrote to the force earlier this month to complain about “police involvement in the Christmas party held at No 10 Downing Street on December 18, 2020”.
Footage of the prime minister’s former spokesperson Allegra Stratton laughing and joking about an alleged gathering was aired on ITV News and led to her resignation.
The Met Police has now confirmed it has referred itself to the Independent Office for Police Conduct (IOPC) in relation to a complaint received about the alleged party.
Lady Jones, a former deputy mayor of London, who was made a life peer in the House of Lords in 2013, wrote on Twitter on Tuesday, December 21: “Update. My complaint against the Met Police is now being investigated by the Independent Office for Police Complaints, [sic] and has additionally been referred to the Mayor’s Office for Policing and Crime.
“Downing Street parties must be investigated! #PartyGate.”
In her complaint, dated December 6, Lady Jones wote: “I believe that there is a case to answer for the police aiding and abetting a criminal offence, or deliberately failing to enforce the law in favour of government politicians and their staff.”
She added that the Met’s “refusal” to investigate “is a conflict of interest and a potential cover-up”.
Sky News reported that acting Det Ch Supt Tony O’Sullivan, directorate of professional standards, told Lady Jones in a letter: "I have referred your complaint to the IOPC 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."
A Metropolitan Police Service (MPS) spokesperson said: “We have received various complaints about the Met’s response to allegations of staff gatherings in Downing Street and elsewhere in November and December 2020.
“Every complaint has been assessed individually. Following this process, one complaint has been referred to the IOPC and MOPAC.
“We are unable to provide details of the complainant or of why that particular complaint has been referred to the IOPC and MOPAC.
“The MPS has diligently followed the regulatory guidelines.”
An IOPC spokesperson said: “We can confirm that, on Friday, December 17, we received a referral from the MPS 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.”
The MOPAC has been contacted for comment.