The sound of The Winner Takes It All is said to have boomed through the building, making it one of the most controversial of up to six events Mr Johnson attended in apparent breach of Covid laws.
Last week, Mr Johnson told Tory MPs it was “a work event” — lifting the lid on the evidence he is likely to have given the police and Sue Gray’s stalled Cabinet Office inquiry.
Now, he is expected to argue that he invited Henry Newman — then an adviser to Michael Gove — to join him in the No 11 flat where he lives with his wife and children.
A government source told The Times that Mr Johnson has claimed he did not break the law because he was conducting a work-related interview with the adviser in another part of the flat.
The PM invited Mr Newman up to the flat to distance the pair from leaving drinks being held elsewhere in the building for Lee Cain, who had just been ousted as his director of communications.
Separately, Downing Street has not denied that the prime minister effectively turned Mr Cain’s event into a leaving party. “He said he wanted to say a few words for Lee and started pouring drinks for people and drinking himself,” a source told the Sunday Times.
Carrie Johnson has previously described her close friend Mr Newman as her political “bestie” and he is said to be a regular Downing Street visitor.
The prime minister is expected to escape a fine for the “bring your own booze” party he attended in the No 10 garden, a legal expert has predicted – despite others being punished.
But his troubles have deepened after he failed to block a Commons inquiry into whether he lied to MPs, when he wrongly told them no Covid rules were broken.
Top Conservatives are predicting that a post-local elections challenge to Mr Johnson’s leadership is now inevitable, as MPs contemplate no end to the controversy.
And, one senior Tory, Steve Baker, has warned the Conservatives will “reap the whirlwind” of public anger over the parties at the ballot box on 5 May.