The transport secretary argued Mr Johnson had only “popped down” to toast departing communications chief Lee Cain on November 13, 2020.
Downing Street has declined to defend the scenes ahead of the publication of Sue Gray’s inquiry, which a No 10 source expects to be published on Wednesday.
The leaving do took place just days after the Prime Minister had ordered England’s second national lockdown.
Mr Johnson, who the transport secretary claimed was “mortified” by the latest revelations, was facing fresh allegations he lied to Parliament after ITV News published the images.
Scotland Yard was also facing calls to explain why Mr Johnson was not fined over that event when photos showed him, drink in hand, by a table strewn with food and wine bottles.
There were at least eight other people in the room at a time when people were banned from social mixing, other than to meet one person outside, and at least one individual has received a fine over an event on that date.
Mr Shapps pointed to Mr Johnson’s red ministerial box being present in the images as he claimed to Sky News the Prime Minister was “clearly not” partying.
“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 it as a party,” Mr Shapps said.
The transport secretary told BBC Radio 4’s Today programme he was “angry” to see the images, but sought to explain why Mr Johnson was not fined over that event.
He said he presumed the Prime Minister “comes in and out pretty quick, which is presumably why the police have not issued a fixed-penalty notice to the Prime Minister for that event”.
London Mayor Sadiq Khan was among those calling for Scotland Yard to explain why Mr Johnson only received a single fine for breaching lockdown rules with his 56th birthday gathering.
The Liberal Democrats have urged the Independent Office for Police Conduct to examine the Met’s Operation Hillman inquiry into events in No 10 and Whitehall.
But Mr Shapps said the Met does not need to explain its rationale, adding: “I don’t think the police should provide running commentaries, no.”
He insisted to ITV’s Good Morning Britain the Prime Minister “did not knowingly lie” to Parliament when he repeatedly denied rule-breaking parties took place in No 10.
Mr Shapps was unable to refute reports suggesting Mr Johnson told Ms Gray in their controversial meeting earlier this month that there is no need for her to publish her report following the police investigation.
Mr Johnson was facing fresh pressure from Conservatives over the latest revelations.
Scottish Tory leader Douglas Ross demanded the Prime Minister explain why he believed his behaviour was “acceptable” when most people would think the pictures showed “unjustifiable and wrong” behaviour.