The prime minister’s former chief aide says he had spoken to people who were in No 10 on November 13, 2020, when an alleged gathering in Johnson’s Downing Street flat took place.
He went on to allege witnesses would say Abba could be heard playing from the apartment above the press office.
In a question and answer session on his paid-for blog, Cummings was asked if there were photos that would “incriminate” the PM.
He said: “Yes there are photos of the PM at parties under investigation. Ive spoken to people who say theyve seen photos of parties in the flat.”
The former adviser added: “Ive talked to people who were in no10 on 13/11 who cd hear the party in no10 after I’d left – the press office is below the flat.
“If cops talk to people there that night, therell be witnsesses (sic) who say ‘we could all hear a party with abba playing’.”
Hours after the Q&A session, reports suggested Johnson attended more gatherings under investigation by the police than had previously been understood.
The Daily Telegraph reported that he was seen heading up to his flat on the night of the “Abba-themed” gathering.
The Telegraph also said a gathering on December 17, 2020, for an official who was leaving was for Captain Steve Higham, then one of Johnson’s private secretaries, who advised on defence and national security issues.
The prime minister was reported to have attended the event and to have given a speech.
He was also reported by both the Telegraph and the Guardian to have been present at a leaving do for two No 10 private secretaries on January 14, 2021.
One of the officials was said to be a senior policy adviser who was moving to the Department for Digital, Culture, Media and Sport.
Again, Johnson was reported to have made a speech thanking the official for their work while some guests drank prosecco.
The two papers also said that another event under investigation in the Cabinet Office on June 18, 2020, was for Hannah Young, a senior official who was leaving No 10 to become deputy consul general in New York.
Downing Street said it could not comment on the latest reports due to the ongoing police investigation.
Officers are investigating 12 separate gatherings in No 10 and Whitehall during 2020 and 2021 – including three that Johnson is known to have attended and the one in the PM’s Downing Street flat – to find out whether coronavirus lockdown laws were broken.
Cummings had accused the PM of “lying” and added: “This could blow up terminally for him if lies to the cops but he wont be able to help himself other than say ‘i dont remember’ which is his default when he senses danger.”
Cummings also said there was “no excuse for self-delusions” in the Tory party about Johnson, adding “at this point the blame lies mostly with the Tory MPs”.
He said: “He’s obviously totally unfit for the job and every day he’s left their moral authority drops another notch.”
But he added: “There’s lots of blame to go around beyond them, including people in no10 who have also shown a distinct lack of moral courage…”
The Ask Me Anything (AMA) event came as the fall-out from the partygate scandal showed no sign calming.
Downing Street said Johnson will reveal if he has been hit with a fine for breaching coronavirus rules, despite the identity of people being issued with a fixed penalty notice not usually being disclosed by police.
Meanwhile, Peter Aldous became the latest Tory MP to publicly call on Johnson to resign “in the best interests of the country, the government and the Conservative Party”.
He said that he believed the Prime Minister had no intention of going voluntarily and so he had submitted a letter of the chairman of the backbench 1922 Committee Sir Graham Brady calling for a vote of no confidence.
The Met is examining hundreds of documents and photographs in relation to the 12 events in 2020 and 2021 held while England was under coronavirus restrictions.
The evidence was passed to the police by the investigation team led by senior official Sue Gray, whose interim report on Monday highlighted “failures of leadership and judgment” at the heart of government but did not point the finger of blame at any individuals.
Her conclusions were limited following a request by the Metropolitan Police to make only limited references to the events under investigation, leaving it to Scotland Yard to decide whether laws were broken.
In his AMA, Cummings said the chances of foreign secretary Liz Truss becoming Tory leader are “probably being overrated”.
He said the fact that “MPs are heavily influenced by polls” would “obviously benefit (Rishi) Sunak” in any future leadership race, and said Sunak pro-Brexit stance would also help him.
Cummings said Truss was “little known” and had “said a lot of stuff that will not be popular with members if/when they hear it”. If she gets to the last 2, the combination of her support for Remain and her record will be big problems for her. Her chances are probably being overrated,” he said.
He said he thought levelling up Secretary Michael Gove “probably will” run again, while on Tory MP Tom Tugendhat, he said: “He has never been in Cabinet and it is hard to imagine Tory MPs promoting someone straight to the top job who has not been in Cabinet.”
He said: “RS is obviously the front runner cos he’s way ahead in the polls and unlike Truss/Hunt/TT was pro-Brexit.
“His team conceived and executed furlough in very tough circumstances, and which was one of most popular things a politician has done in a very long time. That will be a big strength when the contest comes.”