By Guy Faulconbridge and Kylie MacLellan

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

Johnson is facing the gravest crisis of his premiership after almost daily revelations of social gatherings during COVID-19 lockdowns, some held when ordinary people could not bid farewell in person to dying relatives.

As an opinion poll showed the opposition Labour Party pulling into a 10-point lead over Johnson's Conservatives, a report said he had encouraged staff to "let off steam" during regular "wine-time Friday" gatherings.

After building a political career out of flouting accepted norms, Johnson is now under growing pressure from some of his own lawmakers to quit. Opponents say he is unfit to rule and has misled parliament by denying COVID-19 guidance was breached.

In an extraordinary twist to a saga that has been widely lampooned by comedians and cartoonists, the Daily Telegraph said drinks parties were held inside Downing Street on April 16, 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," Johnson's spokesman told reporters.

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 broke a swing used by the prime minister's son.

The next day, Queen Elizabeth bade farewell to Philip, her husband of 73 years, following his death aged 99.

Dressed in black and in a white trimmed black face mask, the 95-year-old Elizabeth cut a poignant figure as she sat alone, in strict compliance with coronavirus rules, during his funeral service at Windsor Castle.

'LEAVE THE STAGE'

Opponents have called for Johnson, 57, to resign, casting him as a charlatan who demanded the British people follow some of the most onerous rules in peacetime history while his staff partied at the heart of government.

A small but growing number in Johnson's 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 lawmaker Andrew Bridgen, a former Johnson supporter. "The time is right to leave the stage."

In the latest report of rule-breaking, the Mirror newspaper said staff had bought a large wine fridge for Friday gatherings, events that were regularly observed by Johnson as he walked to his apartment in the building.

"If the PM tells you to 'let off steam', he's basically saying this is fine," it quoted one source as saying.

Separately, the former head of the government unit behind COVID restrictions, Kate Josephs, apologised for holding her own drinks gathering when she left the job in December 2020.

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.

The Independent newspaper said Johnson had dubbed a plan to salvage his premiership as "Operation Save Big Dog".

Foreign Secretary Liz Truss, seen as a possible successor, said "real mistakes" had been 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 1922 Committee.

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

Johnson faces a tough year ahead: beyond COVID-19, inflation is soaring, energy bills are spiking, taxation will rise in April and his party faces local elections in May.

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