It is understood the apology was made by a Downing Street official, in a telephone call – rather than in 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”.