A Union Jack cake and Marks & Spencer party food were served while 30 guests belted out "Happy Birthday" for Boris Johnson during lockdown, say reports.
The surprise birthday celebration took place on June 19, 2020, despite indoor social events being banned.
The PM's wife, Carrie Johnson, and interior designer Lulu Lytle who had been brought in to decorate the Downing Street flat, brought in the Union Jack cake and they began to sing "Happy Birthday", it is claimed.
Reports from ITV News on the event claim that there was also Marks & Spencer party food while people stayed for around 30 minutes.
The PM is said to have attended the event on the way to chairing a Covid strategy meeting.
A Times source said members of Johnson’s team had been emailed in advance asking them to come to “wish the prime minister happy birthday”.
Martin Reynolds, Johnson's under-fire principal private secretary, was also said to have attended, as was No 10's director of communications Jack Doyle and head of operations Shelley Williams-Walker.
Chancellor Rishi Sunak was understood to have briefly attended as the gathering was breaking up as he entered the room to attend a Covid strategy meeting.
Sue Gray is believed to have already been aware of the birthday party allegations and therefore their emergence will not further delay the publication of her inquiry, which is still expected this week.
The senior civil servant has been investigating a series of claims of rule-breaking parties in No 10 as Johnson faces calls to resign as Prime Minister, including from some of his own Conservative MPs.
No 10 did not deny the event took place, but claimed the prime minister only attended briefly.
A No 10 spokesman said: "A group of staff working in No 10 that day gathered briefly in the Cabinet Room after a meeting to wish the Prime Minister a happy birthday.
"He was there for less than ten minutes.”
The party was held in the Cabinet Room, after Johnson returned from a visit to a school in Hertfordshire.
A group of schoolchildren from Bovingdon School in Hertfordshire had sung "happy birthday" to the PM before this party. They were all standing outside in the school playground.
Social gatherings indoors were forbidden under lockdown laws at the time, with a relaxation of the regulations permitting gatherings of up to six people to take place outside.
Jo Goodman, the co-founder of Covid-19 Bereaved Families for Justice campaign, said: "It's completely sickening that the prime minister spent the evening sharing cake with 30 friends indoors and though we're not even surprised any more, it still brings fresh pain.
"Whilst dozens sang happy birthday to him, families couldn't even sing in memory at their loved ones funerals."
Culture Secretary Nadine Dorries, a staunch ally of Johnson, questioned whether the gathering would have broken the rules in place at the time.
"So, when people in an office buy a cake in the middle of the afternoon for someone else they are working in the office with and stop for ten minutes to sing happy birthday and then go back to their desks, this is now called a party?" she tweeted.
Environment Secretary George Eustice told reporters: "What really happened here is a small group of staff who had been working closely with the Prime Minister brought in a birthday cake at the end of the day and there was 10 minutes there around sharing a piece of cake.
"I don't think that really constitutes a party in the way some of the more serious allegations that are being investigated maybe do."
