UK Prime Minister Boris Johnson witnessed weekly drinking sessions by his staff throughout the pandemic, according to a report Saturday that stoked fresh calls from his own party for the embattled leader to quit.

Conservative MPs, many of whom were spending the weekend back in their home constituencies, say they are being deluged with messages from voters outraged at accounts of rule-breaking in Downing Street.

Johnson was spending the weekend in self-isolation after a family member tested positive for Covid. The opposition Labour party said he was "literally in hiding" and should resign.

Prime Minister Boris Johnson is facing calls to resign over allegations of 'partygate' breaches of Covid rules Prime Minister Boris Johnson is facing calls to resign over allegations of 'partygate' breaches of Covid rules Photo: AFP / Tolga Akmen

Former Conservative minister Tobias Ellwood, an influential backbench MP, told the BBC that Johnson must "lead or step aside".

Whether Johnson and his staff knowingly broke the law during Covid lockdowns is the central question being addressed in an inquiry by senior civil servant Sue Gray, who could report back next week.

After weeks of denials and stonewalling, Johnson this week apologised in parliament for at least one boozy event organised by his staff which he attended in May 2020, when Britons were banned from socialising.

Labour leader Keir Starmer says Prime Minister Boris Johnson is "not fit for office Labour leader Keir Starmer says Prime Minister Boris Johnson is "not fit for office" Photo: UK PARLIAMENT via AFP / JESSICA TAYLOR

Two other parties were held in April 2021 as Queen Elizabeth II prepared to bury Prince Philip, her husband of 73 years. Downing Street sent apologies to Buckingham Palace, calling them "deeply regrettable".

But those were not isolated events, according to Saturday's Daily Mirror, which published a photograph of a wine cooler being delivered to a Downing Street back door in December 2020.

It said staff would stock the fridge with suitcase-loads of alcohol, and Johnson would often drop by to their "Wine time Fridays".

"The idea that he didn't know there were drinks is total nonsense," the newspaper quoted one staffer as saying.

Chart showing approval rating for the government of British Prime Minister Boris Johnson, based on surveys from July 2019 to Jan 10, 2022 Chart showing approval rating for the government of British Prime Minister Boris Johnson, based on surveys from July 2019 to Jan 10, 2022 Photo: AFP / Gal ROMA

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

In response, a Downing Street spokesperson said the government was awaiting Gray's inquiry "to establish the facts around the nature of gatherings" during the pandemic.

Labour leader Keir Starmer says Prime Minister Boris Johnson is "not fit for office Labour leader Keir Starmer says Prime Minister Boris Johnson is "not fit for office" Photo: UK PARLIAMENT via AFP / JESSICA TAYLOR

But at least five Conservative MPs say publicly that they have already filed letters demanding a vote of no confidence in the prime minister.

A total of 54 letters from Tory lawmakers are needed to trigger a vote. The Daily Telegraph newspaper said about 20 have been handed in so far.

After the Mirror report, Conservative backbencher Andrew Bridgen said Johnson had "lost the moral authority to lead".

He had presided over a staff culture of "one rule for them, and the rest of us do as we're told", Bridgen told BBC television.

Most cabinet members have rallied round Johnson but the support of some, including powerful finance minister Rishi Sunak, has been distinctly lukewarm.

Pensions minister Guy Opperman broke ranks to argue that Johnson "needs to change his ways", describing the personal toll inflicted on his family by the Covid rules.

"I feel pretty emotional about this because in May 2020 my wife and kids were unwell and they went to hospital. I was not able to go there to support them," he told the BBC.

The Oppermans' newborn twins died in June 2020.

The drip-feed threatens to overshadow a reported Downing Street plan to salvage Johnson's leadership after the Gray report is released.

The Independent newspaper said "Operation Save Big Dog" would see a clear-out of top Downing Street aides.

The government has already signalled it intends to relax current Covid restrictions in late January and Johnson allies have been touting his achievements in office, including his delivery of Britain's Brexit withdrawal from the European Union.

But the Labour party -- which this week surged to a 10-point lead over the Tories in two opinion polls ahead of local elections in May -- said Johnson was "not fit for office".