Labour has demanded that the Prime Minister account for his “whereabouts” on his 56th birthday after reports emerged that he might have attended a second “potentially illegal” gathering that day.

Boris Johnson was handed a fine by the Metropolitan Police for attending his own surprise birthday bash in Downing Street on June 19 2020 when indoor mixing was forbidden.

In her report published last week, senior civil servant Sue Gray provided details of the event, with sandwiches, cakes and alcohol available for those who attended, including Mr Johnson’s wife Carrie and Chancellor Rishi Sunak who were also given a £50 fixed-penalty notice each.

According to The Sunday Times, there is possible evidence of a second gathering taking place a few hours after the Cabinet room party, with Mrs Johnson said to have been in the Downing Street flat with “several friends”.

The event was not investigated by Ms Gray, the newspaper said, despite her team being informed of it in January.

The paper said an aide possesses messages sent by Mrs Johnson at around 6.15pm appearing to notify her husband, after his aide had informed her the Prime Minister was coming up to the residence, that she was in the No 11 flat “with an unspecified number of male friends”.

The anonymous aide has reportedly since written to Cabinet Secretary Simon Case to inform him of the alleged gathering.

Angela Rayner, the deputy Labour leader, has followed-up by writing to Mr Case calling on him to release correspondence that relates to an alleged evening meet-up.

She told the head of the Civil Service: “It is crucial that you now advise the Prime Minister to come clean about his involvement in this apparently rule-breaking gathering.

“Given the public interest at stake, I request that you make public his correspondence pertaining to this event and his whereabouts on the date of Friday June 19 2020 and publish the relevant messages you have received, as well as handing them over to the Privileges Committee to consider as part of their investigation.”

In her letter, Ms Rayner also asked whether Mr Johnson had “notified” Scotland Yard “of his presence” at a gathering on the evening in question.

She told Mr Case that Downing Street had previously been asked about the evening birthday claims and had branded them “totally untrue”.

The senior opposition figure questioned who authorised that response and whether it would be “retracted on the basis that it was itself either misleading or false”.

The Cabinet Office said it seriously disputed the account of events as detailed in The Sunday Times.

The paper reported that the unnamed aide claimed to have told Ms Gray’s investigation that, while they did not want to forward the messages in their possession, they were prepared to come into the Cabinet Office to show investigators in person.

But the Cabinet Office said the person had not been willing to provide the messages or to meet in person, so their email exchange was forwarded to the police once the Operation Hillman inquiry started.

It is not known whether the police acted on the information but officers, when confirming what events were being investigated, did not list an incident on the evening of the Conservative Party leader’s June 2020 birthday.

The aforementioned aide later came back to Ms Gray’s team but the Cabinet Office said the probe had concluded by the time she approached them a second time.

The Prime Minister, following the conclusion of the Gray and police probes, faces a parliamentary investigation, with the Commons Privileges Committee set to look into whether he misled MPs with his reassurances that Covid rules were followed in No 10.