The revelation ratchets up the pressure on Johnson over a series of gatherings at his 10 Downing Street residence that would seem to have broken the pandemic lockdown rules imposed by his government.

His office disputed the claim it was a party, telling ITV: "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 10 minutes."

ITV said it was alleged the prime minister's wife, Carrie Johnson, helped organise the party on the afternoon of June 19.

Up to 30 people attended the event in the Cabinet Room of No. 10, his office and residence, ITV said. The prime minister was believed to have been presented with a cake whilst his wife led staff in a chorus of happy birthday, it said.

The broadcaster said one of the attendees was interior designer Lulu Lytle, who was renovating Johnson's flat in the building.

ITV also said it also understood that family friends were hosted in the prime minister's residence on the previous evening.

His office denied this claim.

"This is totally untrue," a spokesperson told ITV. "In line with the rules at the time, the prime minister hosted a small number of family members outside that evening."

The birthday allegations add more fuel to a scandal that has engulfed Johnson.

Senior bureaucrat Sue Gray has been investigating the gatherings and is expected to publish a report later this week.

Johnson has given a variety of explanations about previously reported parties: first he said no rules had been broken but then he apologies to the British people for the apparent hypocrisy of such gatherings.

Opposition leader Keir Starmer said of the birthday gathering: "This is yet more evidence that we have got a prime minister who believes that the rules that he made don't apply to him."

"We cannot afford to go on with this chaotic, rudderless government. The prime minister is a national distraction and he’s got to go."