It is understood the apology was made by a Downing Street official, in a telephone call – rather than in a conversation involving the prime minister.
Asked if Mr Johnson knew about the parties on 16 April last year, his spokesman said: “We have set out that the prime minister was not in No 10 in 16 April.”
He also refused to say if the apology to the Palace acknowledged that the gatherings were “social events” – which would have been in breach of Covid rules at the time.
“The facts are for Sue Gray to establish as part of her report,” the spokesman said, referring to the senior civil servant whose report is expected to determine if Mr Johnson can keep his job.
There are no plans for the prime minister to make a statement on the latest allegations of rule-breaking in his Downing Street home, despite Labour taunts that he is “in hiding”.
The fresh crisis erupted after it was revealed that two leaving events were held on 16 April, for the prime minister’s official spokesman and for one of his personal photographers.
About 30 people attended the events in total, witnesses told The Daily Telegraph, with the groups later joining each other in the No 10 garden.
The party for the photographer reportedly involved loud music, a DJ and a staff member sent to the Co-op store on the Strand to fill a suitcase with bottles of wine.
Drinking carried on into the early hours of the morning, witnesses said – and one person broke a swing belonging to Johnson’s infant son, Wilfred.
Keir Starmer seized on the No 10 apology, saying: “This shows just how seriously Boris Johnson has degraded the office of prime minister.
“The Conservatives have let Britain down. An apology isn’t the only thing the prime minister should be offering the palace today. Boris Johnson should do the decent thing and resign.”
Despite the apology, it is unclear exactly what No 10 has said it is sorry for – as it still refuses to discuss the nature of the events that took place during the lockdowns.
The spokesman said: “It is deeply regrettable that this took place at a time of national mourning and No 10 has apologised to the Palace.”
However, he then refused to say what he meant by “this”, telling journalists: “It is right that we don’t pre-empt the findings of the Sue Gray investigation.”
Buckingham Palace declined to comment on the apology it had received.
Downing Street has said that Mr Slack “gave a farewell speech to thank each team for the work they had done to support him” – but has refused to comment on the other leaving do.
Mr Johnson’s spokesman said he was at his country residence Chequers on 16 April last year and had not been invited to the events.
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