The Met Police has admitted it did not send Boris Johnson questionnaires about two lockdown-busting gatherings at Downing Street for which other attendees were fined.
The Prime Minister did not face questions from the force over his involvement in the events, despite Sue Gray finding he attended both.
The Mirror revealed earlier this year that Mr Johnson would only be investigated for attending two social events in Downing Street when they were banned - even though he was believed to have attended six.
The PM was informed by Scotland Yard that he would be subjected to a limited probe as part of the four-month inquiry, we reported.
The revelation is thought to be the first time the Metropolitan Police has admitted this, under details released as part of a legal challenge by the Good Law Project.
Mr Johnson received a fixed-penalty notice over a birthday party in the Cabinet Room in June 2020, but faced no further action over other gatherings covered by the Met's Operation Hillman inquiry.
The Good Law Project criticised the Met for continuing "to fail to provide any explanation of how they cleared the Prime Minister".
The force issued 126 FPNs to 83 people at events in Downing Street and Whitehall, including to other attendees at both the November 13 2020 and December 17 2020 gatherings.
"We don't think the Met's response is consistent with their legal duty of candour. And we certainly don't think it's consistent with what the Met has elsewhere conceded is their public duty to maintain public confidence in policing," the campaigners said.
"The public has a right to know what really went on inside the Partygate investigation. The Met's actions have raised grave concerns about the deferential way in which they are policing those in power".
The group vowed to proceed with a judicial review.
Scotland Yard said: "The Op Hillman investigation has concluded and the MPS will not comment on investigative steps taken in the course of that investigation.
"We can confirm that the MPS has filed submissions in response to an application filed at the High Court seeking a judicial review of decision-making pertaining to the Op Hillman investigation".
London Mayor Sadiq Khan has urged the force to explain its decisions over Partygate, while the Met has also been asked to consider a Liberal Democrat complaint about its handling of the probe after the pictures emerged of Mr Johnson at the November 13 2020 event for which he was not fined.
