London mayor Sadiq Khan has asked police for an explanation after “smoking gun” pictures emerged of Boris Johnson drinking at an event during lockdown – for which he was not fined.

Mr Khan, who oversees policing in the capital, told Sky News’ Kay Burley that the pictures from a Downing Street leaving do in November 2020 appeared to show the PM “caught red-handed”.

Grant Shapps, the transport secretary, told Sky News that Mr Johnson was “not partying” in the photographs, which were obtained by ITV News.

Politics live: Boris Johnson suggested Sue Gray drop partygate report – source

Police are understood to have issued fines over the event as a result of their investigation into lockdown-breaking events in Downing Street and Whitehall.

The prime minister himself received a fixed penalty notice in relation to a separate gathering on his birthday in June 2020 but at the conclusion of the Met Police probe last week it was disclosed that he had not been fined for any other events.

Mr Khan said: “There clearly was a party – he was raising his glass, there were empty wine bottles there… I think the police should be explaining how they reached their conclusions, that’s the right thing for them to do.”

He said he was asking police for “urgent clarity”.

“People across the country will be saying, not unreasonably – ‘hold on a second, I followed all the rules, I was meticulous in relation to following the rules’,” he said.

“Those that broke the rules were fined and here we have a photograph which looks like the prime minister caught red-handed breaking the rules – some would call it a smoking gun.

“So the police should explain how it is more fines weren’t issued.”

The newly-released pictures of the PM are of a gathering to mark the departure of the PM’s then communications chief Lee Cain.

Mr Shapps told Sky News: “The question is, was he down there partying? No, clearly not – he’d gone by to say thanks and raise a glass to a colleague who was leaving.”

“The police have spent a lot of time with a lot of people and a lot of resources crawling over it and they’ve come to their conclusion – as we know he wasn’t fined for that event.”

But Mr Shapps was also pressed on the prime minister’s assurance to parliament, when asked about the gathering, that he was sure that “the guidance was followed and the rules were followed at all times”.

The transport secretary said that the PM’s ministerial red boxes could be seen.

“It looks to me like he goes down on his way out of the office and thanks the staff and raises a glass and doesn’t in his mind recognise that as a party,” he said.

Mr Shapps was also asked about the controversy swirling over the PM’s meeting with senior civil servant Sue Gray, details of which were first reported by Sky News last week – ahead of the expected publication this week of her full report into the partygate affair.

A source has told Sky News that in the meeting Mr Johnson implied Ms Gray should consider dropping the report – questioning what more was left to say given that the police investigation had concluded – though this is contested by his allies. The claim was first reported by The Times newspaper.

Mr Shapps said he was not at the meeting, which he said was to arrange “timing and logistics” but said that “sometimes occasionally things get reported that are not entirely accurate”.