A fixed-penalty notice to be issued to Boris Johnson appears to relate to a surprise birthday party held for the Prime Minister in the Cabinet Room at Number 10.

A spokesperson for the Prime Minister’s wife has also said Mrs Johnson has paid a fixed-penalty notice relating to a gathering on the afternoon of June 19 2020 and “apologises unreservedly”.

Chancellor Rishi Sunak has also been fined, but it has not been confirmed which incident that relates to.

But Mr Sunak has previously confirmed he was in the Cabinet Room that day, but said he attended for a Covid meeting.

The Prime Minister was “in a sense, ambushed with a cake” at the event, Northern Ireland minister Conor Burns has previously said when defending Mr Johnson.

Although Mr Burns later said he was told “under some authority, indeed from him, that there actually wasn’t a cake”.

Downing Street has previously admitted staff “gathered briefly” in the Cabinet Room on June 19 2020, in what was reportedly a surprise get-together for the Prime Minister organised by Carrie Johnson.

The PA news agency was told Lulu Lytle, the interior designer behind lavish renovations of the Downing Street flat, briefly attended while undertaking work there.

However, No 10 has denied a report that, later the same evening, family and friends were hosted upstairs to celebrate the occasion.

Mr Johnson’s birthday is on June 19.

Providing further clarity on the incident leading to the fine, a spokesman for No 10 said: “The Met Police have now explained that the FPN issued to the PM will be in relation to the following incident: On June 19 2020 at the Cabinet Room, 10 Downing Street, between 1400 and 1500 you participated in a gathering of two or more people indoors.”

The incident hit the headlines in this year, when ITV News reported Mrs Johnson had organised the surprise get-together complete with a chorus of “happy birthday” on the afternoon of June 19 2020.

Mr Burns then offered his “ambushed with a cake” defence when speaking about the allegations to Channel 4 News.

He said: “It was not a premeditated, organised party in that sense, that the Prime Minister himself decided to have.

“As far as I can see, he was, in a sense, ambushed with a cake.

“They came to his office with a cake, they sang Happy Birthday, he was there for 10 minutes.

“I don’t think most people looking at that at home would characterise that as a party.”

Mr Burns later told The Telegraph’s Chopper’s Politics podcast: “I’m told under some authority, indeed from him, that there actually wasn’t a cake”

A resurfaced article from The Times the day after the gathering suggested there had been cake – and it was decorated with a Union Jack pattern.

Downing Street conceded staff “gathered briefly” in the Cabinet Room.

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.

Responding to reports of the incident in January, Transport Secretary Grant Shapps said: “The Prime Minister clearly didn’t organise to be given a cake.

“Some people came forward and thought it would be appropriate for on his birthday.”

Soane Britain, the luxury designer co-founded by Ms Lytle, said she was “present in Downing Street on June 19 working on the refurbishment” to Mr Johnson’s flat.

“Lulu was not invited to any birthday celebrations for the Prime Minister as a guest,” a spokeswoman said.

“Lulu entered the Cabinet Room briefly as requested, while waiting to speak with the Prime Minister.”

Asked about the incident in February this year, the Chancellor said: “I am in the Cabinet Room for a Covid meeting much like the other 100, 200, God knows how many other Covid meetings.”

“You’re asking me about something that happened almost two years ago.

“I went to that Cabinet Room, as I did many other times for Covid meetings.

“And it’s right that we allow this police investigation,” he said.

ITV news also reported that later on in the evening on June 19 2020 family friends were hosted upstairs to further celebrate the Prime Minister’s 56th birthday in his official residence.

Downing Street said at the time: “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.”