London Mayor Sadiq Khan has asked the Met Police for a "detailed explanation" of how it decided who to fine during its investigation into No 10 parties.
The mayor's spokesperson said Mr Khan was concerned a "lack of clarity" was eroding "trust" in the police.
The police completed their inquiry last week having issued 126 fines, including one for the prime minister.
However, photos of Boris Johnson at a leaving do have raised questions about why he did not receive a second fine.
Pictures obtained by ITV News are believed to show the prime minister at a leaving party for his communications chief Lee Cain on 13 November 2020.
The BBC has been told that at least one person who attended the event was fined, but the PM was not.
Transport Secretary Grant Shapps said the pictures were "difficult to look at" but added that the police would have "thoroughly investigated" the event.
Following the release of the photographs on Monday evening, the Liberal Democrats wrote to the Independent Office for Police Conduct (IOPC) asking the body to investigate the police's inquiry.
The IOPC confirmed it had received the complaint, and had sent it to the Met, asking the force "to confirm to us how it intends to handle it".
Now, Labour London Mayor Mr Khan has written to the acting commissioner of the Met, Sir Stephen House, to "seek a detailed explanation of the factors which were taken into account by investigating officers when decisions were made about whether to take action in individual cases in the Downing Street 'Partygate' investigation".
His spokesman said he had asked the police "to take steps to also reassure Londoners by making this explanation to them directly, because he is concerned that the trust and confidence of Londoners in the police is being further eroded by this lack of clarity".
"The mayor has been clear he cannot and would not intervene in operational decisions, however with the investigation now complete, he has made this request in accordance with the Policing Protocol Order 2011 paragraph 23(g)."
A No 10 spokeswoman has said the police had been given access to information, including photographs, for its investigation.
The inquiry concluded, with a total of 126 fines - also called fixed penalty notices - being issued to 83 individuals for parties held over eight separate dates.
Last month, the police issued fines to Mr Johnson, his wife Carrie and Chancellor Rishi Sunak for attending a birthday party for the prime minister in June 2020.
The prime minister apologised and said "people had the right to expect better".
However, he is facing continued questions, after one insider who attended lockdown events told the BBC's Panorama they felt like they had the prime minister's permission to socialise even it meant breaking the rules because "he was there".
A long-anticipated report from senior civil servant Sue Gray into rule-breaking events in government buildings during the coronavirus lockdown is expected to be published soon.
A government source has also told the BBC there will be a statement to Parliament and a press conference by the prime minister, once the report is published.
Mr Johnson is also expected to address a meeting of his backbenchers at the 1922 Committee.