London mayor Sadiq Khan has written to the Met Police asking for a “detailed explanation” of its partygate probe – after claiming photos showed the PM was “caught red-handed” at an event for which he was not fined.

Mr Khan said he had written to the Metropolitan Police’s acting commissioner Sir Stephen House to ask how decisions were made about whether fines should be issued in individual cases.

Earlier, the mayor told Sky News that newly-released photos showing Boris Johnson at a leaving do in November 2020 were a “smoking gun”.

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A spokesman for the Met said it had nothing to add to the statement issued last week when it said the investigation into lockdown-breaking events in Downing Street and Whitehall had concluded, with 126 case involving 83 people being referred for fixed penalty notices.

Number 10 said it would not be commenting.

A spokesperson for the mayor – who is responsible for policing in the capital – said Mr Khan had written asking for an 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 probe.

“He has asked them 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 spokesperson said.