British Prime Minister Boris Johnson's office has apologised to Queen Elizabeth II after it emerged that staff had partied late into the night in Downing Street on the eve of Prince Philip's funeral, at a time when mixing indoors was banned.

Mr Johnson is facing the gravest crisis of his premiership after almost daily revelations about a series of social gatherings during Covid-19 lockdowns, some held when the public could not bid farewell in person to dying relatives.

The British prime minister is now under growing pressure from some of his own politicians to resign.

Opponents say he is unfit to rule and has misled parliament by denying Covid-19 guidance was breached.

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The Daily Telegraph said drinks parties were held inside Downing Street on 16 April, 2021, the day before Prince Philip's funeral.

"It is deeply regrettable this took place at a time of national mourning and No 10 (Downing Street) has apologised to the Palace," Mr Johnson's spokesman told reporters.

A spokesperson said Boris Johnson was at Chequers on the day

Mr Johnson was at his Chequers country residence that day and was not invited to any gathering, his spokesman said.

Such was the revelry in Downing Street, the Telegraph said, that staff went to a nearby supermarket to buy a suitcase of alcohol, spilled wine on carpets, and a swing used by the prime minister's young son was broken.

The next day, Queen Elizabeth II bade farewell to Prince Philip, her husband of 73 years, following his death aged 99. She cut a poignant figure as she sat alone, in strict compliance with coronavirus rules, during the funeral service at Windsor Castle.

Queen Elizabeth II at her husband's funeral

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Johnson faces calls to resign

Opponents have called for Mr Johnson to resign, casting him as a charlatan who demanded the British people follow some of the most onerous rules in peacetime history while his own staff partied at the heart of the British state.

A small but growing number in his own Conservative Party have echoed those calls, fearing it will do lasting damage to its electoral prospects.

"Sadly, the prime minister's position has become untenable," said Conservative politician Andrew Bridgen, a former Johnson supporter. "The time is right to leave the stage."

Mr Johnson has given a variety of explanations of the parties, ranging from denials that any rules were broken to expressing understanding for the public anger at apparent hypocrisy at the heart of the British state.

British Foreign Secretary Liz Truss, seen as a possible successor, said "real mistakes" were made.

"We need to look at the overall position we're in as a country, the fact that he (Johnson) has delivered Brexit, that we are recovering from Covid... He has apologised."

"I think we now need to move on."

To trigger a leadership challenge, 54 of the 360 Conservative members of parliament must write letters of no confidence to the chairman of the party's so-called 1922 Committee.

The Telegraph said as many as 30 such letters had been submitted.

Police officers on patrol outside No 10

Mr Johnson faces a tough year ahead: beyond Covid, inflation is soaring, energy bills are spiking, taxation will rise in April and his party faces local elections in May.

One of the April 2021 parties was a leaving event for James Slack, a former director of communications at Downing Street, who today apologised "for the anger and hurt caused".

Mr Slack, now deputy editor of the tabloid Sun newspaper, said in a statement to PA Media that the gathering "should not have happened at the time that it did".

British police said yesterday they would not investigate gatherings held in Mr Johnson's residence during a coronavirus lockdown unless an internal government inquiry finds evidence of potential criminal offences.