British Prime Minister Boris Johnson was embroiled Tuesday in another scandal over his government's alleged lockdown breaches, as police said they were investigating a Downing Street gathering to which over 100 people were invited.

In the latest in a string of such accusations, Johnson and others allegedly held a drinks party in the garden of his official residence in May 2020 when all in-person socialising was outlawed.

An email leaked late Monday indicated that Martin Reynolds, a senior civil servant, invited more than 100 Downing Street colleagues to "bring your own booze" to the event, which Johnson and his wife Carrie allegedly attended.

The potentially highly damaging revelations follow a series of similar claims which emerged last month about Downing Street parties held during later lockdowns in the run-up to Christmas in 2020.

They prompted Johnson to appoint another senior civil servant, Sue Gray, to investigate and she is now expected to expand her probe to cover the new allegations.

Meanwhile in a statement late Monday, London police said they were also 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.

- 'Socially distanced drinks' -

Johnson has previously denied knowledge that any rules were broken in Downing Street during the pandemic, 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 sent by Reynolds, he 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."

Britain at the time was in the throes of its first lockdown, and outdoor social gatherings of any kind were banned.

ITV News, which obtained the email, said around 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.

"It's right that Sue Gray is looking into this matter independently," Health minister Ed Argar told Sky News on Tuesday during a round of broadcast interviews.

"I'm not going to make comments that would prejudge or get in the way of that."

Come clean

However, the new claims drew a chorus of condemnation from political opponents and others, heaping fresh pressure on the embattled leader after a series of separate sleaze scandals.

"Boris Johnson, your deflections and distractions are absurd," tweeted Keir Starmer, leader of the main opposition Labour party. 

"Stop lying to the British public. It's time to finally come clean."

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

"We have seen a whole series of issues over the last few months," he said Tuesday.

"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."

Johnson, elected by a landslide in December 2019, had hoped to start the new year afresh, leaving behind the so-called "partygate" scandals and other claims of cronyism and corruption that have put his position under threat.

But many of Tuesday's newspapers, including those which normally back Johnson and his Conservative party, again splashed the latest revelations over their front pages.

"Enough Boris! You must end 'partygate' farce now" implored the typically supportive Daily Express.