The prime minister’s wife Carrie Johnson led the surprise gathering at No 10 on the afternoon of 19 June which featured up to 30 people, ITV News has reported.
She presented him with cake and led staff in a chorus of happy birthday, before attendees enjoyed picnic food from M&S at the event.
A No 10 spokeswoman confirmed that a group of staff had “gathered briefly” in the Cabinet Room “to wish the prime minister a happy birthday”, adding: “He was there for less than ten minutes.”
ITV News also reported family friends were hosted upstairs in the prime minister’s flat for an event later that evening – a claim denied by No 10.
“This is totally untrue. In line with the rules at the time, the prime minister hosted a small number of family members outside that evening,” a Downing Street spokeswoman added.
Sir Keir Starmer responded to the latest claim of a rule-breaking party by saying Mr Johnson had become a “national distraction”, adding: “He’s got to go”.
The Labour leader added: “We’ve got a prime minister and a government that spends their whole time mopping up sleaze and deceit, meanwhile while millions of people are struggling to pay their bills.”
Interior designer Lulu Lytle – the person later caught up in the scandal over the complex funding of the PM’s flat refurbishment – also attended the afternoon birthday gathering, ITV reported.
In June 2020 all social gatherings indoors were still banned under Covid laws. Mr Johnson had asked the public to “show restraint and respect the rules” during a press conference nine days before his birthday gathering.
Following the latest allegations, many people shared a letter Mr Johnson wrote to a seven-year-old girl who had written to him to say she delayed her own birthday party in March 2021 because of Covid rules.
“Josephine sets a great example to us all by postponing her birthday party until we have sent coronavirus packing,” the PM tweeted at the time – urging the public to “be like Josephine”.