Pressure on the prime minister over the parties has stepped up after it emerged that his top communications adviser Jack Doyle handed out awards to staff at a gathering on 18 December.
Mr Cummings claimed Mr Johnson was “lying” about the gatherings and said photo evidence of the events – as well as invitations sent to people who do not work at No 10 – would soon be revealed.
“There’s lots of pictures of the parties which will inevitably get out. And invite lists beyond No 10, to other departments,” he tweeted on Friday.
The former strategist also said he thought Mr Johnson probably knew about the 18 December event at the centre of the scandal, and predicted the PM would “be gone before the next election … probably summer”.
In a question and answer event for subscribers to his blog, Mr Cummings also suggested that Mr Doyle would become the fall guy for the party scandal – claiming the press chief is a “gonner” (sic).
He claimed Mr Johnson “will be thinking ‘not now, gotta keep him as the sacrifice for Case’s inquiry then – I’m shocked, shocked I tell you to discover there was a party and I was misled’.”
Mr Cummings also suggested the PM – facing accusations that he lied to his own ethics adviser about donations spent on Downing Street flat redecorations – would “do a deal with Jack to keep all the wallpaper horror buried”.
Referring to the events of 18 December, Mr Cummings also suggested Mr Johnson may have known about the gathering of staff. “He knew but I think did not attend, though remember the geography – to get upstairs he has to walk past that area where he could see it – if he was not [at] Chequers.”
Mr Cummings also alleged that Mr Johnson, whom he refers to as the “trolley”, had been “lying” when answering questions about the alleged parties in recent days.
He claimed: “There were invites sent across Whitehall, it was an organised party. The trolley knows this and tried to lie his way out but was fck [sic] by the video.”
The former No 10 strategist also denied claims he was the source of the leaked video which showed adviser Allegra Stratton, who has since resigned, joking about a festive party. Asked if he obtained the clip and shared with ITV News, he replied: “No.”
Asked by subscribers how and when the Tories might replace Mr Johnson, Mr Cummings said: “The polls will lead it, plus his inevitable continued flat spin, plus officials kicking him off the ice, plus rivals strategically intervening.”
“He’s done, gone by this time next year, probably summer,” he said of Mr Johnson’s leadership – warning the 2019 intake of so-called red wall Tory MPs that they would be “toast” unless they replace him.
But Mr Cummings would not be drawn on who would be the next PM. “Don’t want to get into personalities now – [Liz] Truss and [Sajid] Javid are not solutions.” Asked if his former Vote Leave campaign ally Michael Gove was the answer, he said: “No.”
It comes as Tory peer Gavin Barwell said the party’s MPs are “definitely” having conversations on how to replace Mr Johnson. “His position depends on being seen as an electoral asset, and if over a period of time that goes then he really is in trouble.”
Mr Johnson’s troubles were made worse on Friday when it emerged that his ethics adviser Lord Geidt has restarted an investigation into the funding of the lavish flat refurb.
Lord Geidt contacted No 10 after Thursday’s Electoral Commission report appeared to show the prime minister “misled” the adviser on when exactly he knew about funding arrangements.