The Met Police will be sued in the High Court after the force was accused of not properly investigating Boris Johnson over Partygate.
Formal proceedings were issued today by Brian Paddick, the Lib Dem peer and Scotland Yard's former Deputy Assistant Commissioner, and the Good Law Project, which took a string of legal actions against the government over Covid.
Their case accuses the Met of failing to issue questionnaires to Boris Johnson over three Covid law-breaking leaving dos that were held at 10 Downing Street.
They were for aide Lee Cain on 13 November 2020, for defence advisor Capt Steve Higham on 17 December 2020, for two No10 private secretaries on 14 January 2021.
The Mirror previously revealed that Boris Johnson was only sent questionnaires for two out of the six shindigs he attended.
They were for his 56th birthday party on June 19, 2020 - in which he, wife Carrie and Chancellor Rishi Sunak each received fixed penalties - and a gathering on 13 November 2020 in the Downing Street flat.
The case alleges that, because other questionnaires were not sent, the Met's decision that the PM did not break the law at these gatherings was "irrational" and based on an "error of law".
The Met "failed to seek out any explanation or representations from the Prime Minister", the High Court documents say.
The Met has until July 22 to respond to the legal proceedings before the next stage.
It came hours after the investigation into whether Boris Johnson deliberately misled MPs over Partygate officially began.
Labour veteran Harriet Harman was confirmed as chair after the Privileges Committee finally met for the first time over the probe - two months after MPs ordered it.
Crucially, the Committee have agreed they will be wiling to take evidence from people who want to remain anonymous - such as civil servants in No10.
The final report by the committee, expected no earlier than October or November, is widely regarded as a moment of high peril for the Prime Minister.
Tory rebels have already been plotting his downfall after humiliating defeats in two crunch by-elections last week.
It is the second time the Good Law Project has sued the Met - the first was when the force was refusing to investigate Partygate entirely.
It comes as a day after inspectors placed Scotland Yard under special measures following a series of scandals.
Lord Paddick said: "Members of the public will have seen Boris Johnson raising a glass at a party which he apparently hasn't been questioned about.
“I thought, 'If that had been me, I would have been fined.'
“We are determined that the Prime Minister should be held to the same standard as the rest of us."
Jo Maugham, Director of Good Law Project said: “The public’s faith in the Met has been severely compromised this year - it has failed to hold the Prime Minister and those around him to account for their lockdown breaches, and there have also been shocking reports of institutional misogyny, discrimination and sexual harassment.
“This is its moment to finally begin repairing the damage created by the Met’s inaction and restore the public’s trust.”
A Met Police spokesperson said: "We are aware an application has been filed and we will respond to that application in due course."
