Downing Street has admitted that staff gathered inside No 10 to celebrate Boris Johnson's birthday when the first Covid lockdown was still in place.
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 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."
A source told the BBC this second gathering had involved a barbecue in the No 10 garden with members of Mr Johnson's family.
A source told the BBC that the prime minister's then-fiancee, 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 spokeswoman for Ms Lytle's company said she was present in Downing Street on 19 June working on the refurbishment, and "entered the Cabinet Room briefly as requested, while waiting to speak with the prime minister".
"Lulu was not invited to any birthday celebrations for the prime minister as a guest," the spokeswoman added.
Environment Secretary George Eustice denied that 30 people had gathered in the Cabinet Room, saying the number of staff present was "closer to 10".
Speaking to broadcasters, he said: "My understanding is it was literally just a birthday cake brought in at the end of the day.
"I don't think that really constitutes a party in the way that some of the more serious allegations that are being investigated maybe do.
"I think some of these allegations around parties have got out of hand".
What were the rules in England on the PM's birthday?
- Groups of more than two people were banned from socialising indoors, with up to six allowed outdoors
- People no longer needed a "reasonable excuse" to be outside, but needed one to stay overnight away from home
- Police could no longer use force to break up a prohibited gathering taking place in private
- Non-essential shops had reopened four days earlier on 15 June, with those living alone or single parents able to form a support bubble
- Pubs, restaurants, hairdressers and other leisure facilities were still closed, ahead of reopening on 4 July
- Two households were allowed to meet indoors from 4 July, and weddings with up to 30 guests were allowed
The BBC has learned that Sue Gray, the senior civil servant compiling a report into gatherings on government premises during Covid restrictions, already knew about the 19 June event.
Her findings are expected to be published later this week.
Labour leader Sir Keir Starmer said: "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.
"And so we have got a prime minister and a government who spend their whole time mopping up sleaze and deceit."
Sir Keir repeated his call for Mr Johnson to resign.
Jo Goodman, co-founder of the Covid-19 Bereaved Families for Justice group, did the same, saying of 20 June 2020: "It was the day before what would have been my dad's 73rd birthday, shortly after he had passed away from Covid-19.
"It was a horrible time for my family, but we stuck to the rules, not even being able to hug to comfort each other."
She added it was "completely sickening" that No 10 staff had gathered indoors to eat cake for the PM's birthday, saying: "While dozens sang Happy Birthday to him, families couldn't even sing in memory at their loved ones' funerals."
Some Tory MPs, including the party's Scottish leader Douglas Ross, have called on Mr Johnson to go, but many more say they are waiting for Ms Gray's findings before making a judgement.
At total of 54 MPs must write to Sir Graham Brady, chairman of the backbench 1922 Committee, declaring no confidence in the PM, to trigger a leadership contest.
Former Conservative Party chair Baroness Warsi told the BBC News Channel it was time for Mr Johnson to "think long and hard about what is in the best interest of this country".
"The question he should be asking himself every morning is, 'Is me staying in office allowing me to run this office in a way in which is making the country better, or am I a distraction?'" she added.
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