ITV News reports up to 30 people attended the June 2020 event, sang Happy Birthday and were served cake.
No 10 said staff had “gathered briefly” to “wish the prime minister a happy birthday”, adding that he had been there “for less than 10 minutes”.
Rules at the time banned most indoor gatherings of more than two people.
ITV News said the birthday event had taken place in the Cabinet Room just after 2pm on 19 June 2020, adding that it had been a surprise for Mr Johnson after he returned from an official trip to a school in Hertfordshire.
It also reported that family and friends had been hosted in the prime minister’s residence that evening.
But a No 10 spokesperson said: “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.”
And a source also told the BBC this second gathering had involved a barbecue in the No 10 garden with members of Mr Johnson’s family.
Under the Covid restrictions in place at the time, up to six people could gather outside, in a socially distanced manner.
But gatherings inside of two or more people were still not permitted indoors, except for certain reasons such as work or education.
A source told the BBC that the prime minister’s then fiancée, Carrie Symonds, had organised the cake served in the Cabinet Room and arrived at the event with Lulu Lytle, the interior designer who carried out a revamp of the couple’s Downing Street flat.
They also said some sandwiches and picnic food had been arranged and that there was no discussion about social distancing, with up to 30 people in attendance.
A blatant disregard of the rules at the time …
BJ might try to play it down but plainly Ms BJ cares as much about rules as he does - they’re a pair of self-obsessed parasites and their attitude has, evidently, been adopted by their “staff” …
The numbers involved will, hopefully, be revealed by No 10’s security records …
This latest exposé will surely be the straw that breaks the camel’s back …