UK Prime Minister Boris Johnson on Tuesday faced a furious backlash over new claims of a breach of coronavirus restrictions by his team on top of a slew of recent scandals.

London's Metropolitan Police said they were in contact with the Cabinet Office about the May 2020 gathering in the garden of Johnson's Downing Street residence and office, raising the possibility of a more serious, criminal probe.

More than 100 people reportedly received an emailed invitation to the drinks party, at which guests, allegedly including Johnson and his wife Carrie, were encouraged to "bring your own booze".

At the time, the government was ordering ordinary members of the public not to meet, even outdoors, and tight restrictions were in place on social mixing, including at funerals.

"It truly is beyond belief that while the rest of the country was in lockdown, Johnson and his staff felt it was acceptable to have a party," Rivka Gottlieb, of the Covid-19 Bereaved Families for Justice group, told Sky News.

British media and social networks were filled with recollections from people who said they stuck to the rules, even as family and friends were sick or dying from Covid.

Police at the time fined those breaching the rules, and had the option to prosecute repeat or egregious offenders.

The main opposition Labour party's leader Keir Starmer accused Johnson of "lying to the British public" while deputy leader Angela Rayner said he was dodging questions that went to the heart of his "honour and integrity".

Johnson, elected by a landslide in December 2019, had hoped to start the new year afresh, leaving behind previous revelations of other lockdown-breaking parties at Downing Street in 2020, and claims of cronyism and corruption.

The prime minister's official spokesman refused to comment but said the senior civil servant who sent the May invitation, Martin Reynolds, remained in post and retained the confidence of the prime minister.

Previous claims of Downing Street parties prompted Johnson to appoint Sue Gray, another senior civil servant, to investigate. Her probe has now been widened to include the latest allegations.

Prime Minister Boris Johnson's principal private secretary Martin Reynolds is accused of inviting guests to a garden party at Downing Street during coronavirus restrictions in May 2020
Prime Minister Boris Johnson's principal private secretary Martin Reynolds is accused of inviting guests to a garden party at Downing Street during coronavirus restrictions in May 2020 AFP / Justin TALLIS

"It will establish the facts and if wrongdoing is established there will be requisite disciplinary action taken," minister Michael Ellis told MPs.

But with some opposition MPs calling for Johnson to quit, or be forced out, Ellis said: "The prime minister is going nowhere. The prime minister retains the confidence of the people of this country."

Jonathan Evans, head of the independent Committee on Standards in Public Life, which advises ministers on ethics, said the latest claims showed government "carelessness" over maintaining standards.

"People care and feel that those people who are representing them in parliament, those people who are being paid to undertake public roles, should be living up to the standards that they profess to live up to," he said.

Newspapers, including those which normally back Johnson and his Conservative party, also went on the attack.

London's police said late Monday they were making enquiries over potential breaches of the lockdown laws in relation to the May gathering.

"The Metropolitan Police Service is aware of widespread reporting relating to alleged breaches of the Health Protection Regulations at Downing Street on May 20, 2020 and is in contact with the Cabinet Office," the force said.

Johnson has previously denied knowledge that any rules were broken in Downing Street, as he faced weeks of excoriating headlines over the previous allegations before Christmas.

But the latest accusations appear to directly contradict those claims.

In the May 2020 email, Reynolds wrote: "After what has been an incredibly busy period it would be nice to make the most of the lovely weather and have some socially distanced drinks in the No10 garden this evening."

ITV News, which obtained the email, said some 40 staff ended up gathering in the garden that evening, eating picnic food and drinking.

BBC News said it had contemporaneous emails from some Downing Staff that questioned the wisdom of the invitation.