Boris Johnson is facing calls to come to the Commons to explain how he came to attend a lockdown-busting "bring your own booze" party in the Downing Street garden.
As well as a Whitehall inquiry into allegations of several Downing Street parties during lockdowns in England, the prime minister could now also face a police investigation.
After Opposition leaders backed a police probe into a party held on 20 May 2020, Scotland Yard announced that the Metropolitan Police is considering investigating the event.
Labour's deputy leader Angela Rayner is also demanding that the PM appears before MPs to answer claims that he misled the Commons in previous denials about Number 10 parties.
Possible police investigation
Scotland Yard's announcement of a possible police investigation followed the disclosure of an email from senior No. 10 official Martin Reynolds inviting more than 100 Downing Street staff to a party in the garden.
He wrote: "Hi all. After what has been an incredibly busy period we thought it would be nice to make the most of this lovely weather and have some socially distanced drinks in the No10 garden this evening."
And then, in a highly incriminating plea, he added: "Please join us from 6pm and bring your own booze! Martin."
After it emerged that Mr Johnson and his wife Carrie attended the party, Scotland Yard said: "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 blame game begins
Mr Reynolds, the PM's principal private secretary, has been tipped by MPs as the fall guy for the party fiasco, a view reinforced by a blistering attack on him by a senior government source which appears to make his position untenable.
The source told Sky News Mr Reynolds' job was to protect No. 10 staff from the spread of COVID and he had failed to do that. "The public are right to be outraged at the 'one rule for them, one rule for us'," the source said.
Claiming some people in No. 10 felt uncomfortable about the party at the time, the source added: "He was in charge of it all. He has to be held accountable for it."
The Daily Telegraph reports that a number of Downing Street figures had branded the event a "mad idea" at the time and that Mr Reynolds was the "gatekeeper" responsible for ensuring compliance with COVID rules.
Backing a police investigation, the Liberal Democrats' Home Affairs spokesperson Alistair Carmichael said: "This garden party looks like a clear breach of lockdown rules at a time the public were being ordered to stay at home.
"Boris Johnson himself said just days after this party that the police should step in to stop people holding outdoor gatherings. It would absolutely reek of double standards if the police don't investigate these allegations in full.
"We must get to the bottom of this. If rules were broken, those responsible should face the full force of the law."
PM's House of Commons statements on parties
And Ms Rayner has now signalled that Labour will attempt to force Mr Johnson or another government minister to appear in the Commons to answer questions about Downing Street parties.
"The series of revelations about parties and other events at Downing Street have made a mockery of all who diligently followed the rules - often at great personal cost," she said.
"Boris Johnson repeatedly claimed that no rules were broken in Number 10 but the truth is out. He misled the public and misled the House.
"Of course it's right there's an inquiry but we shouldn't need that for the prime minister to tell the truth.
"Boris Johnson should come to the House of Commons today and finally come clean."
Labour wants to challenge the PM on his repeated denials that lockdown-breaking parties have been held in Downing Street
On December 1 he told MPs during Prime Minister's Questions: "All guidance was followed completely in No. 10." And on December 7 he said: "All the guidelines were observed."
At PMQs on December 8 he said: "I have been repeatedly assured since these allegations emerged that there was no party and that no COVID rules were broken."
And in an interview on December 13, he said: "I can tell you once again that I certainly broke no rules. All that is being looked into."
Asked just hours before Mr Reynolds' "bring your own booze" email was revealed, Mr Johnson was asked: "Did you and Carrie attend a Downing St party on May 20?"
He replied: "All that as you know is the subject of a proper investigation by Sue Gray."
What were the rules on 20 May 2020?
The COVID rules in May 2020, in a month of record temperatures, were spelled out by the then Culture Secretary, now Tory party chairman, Oliver Dowden only an hour before the 20 May party began.
"You can spend time outdoors and exercise as often as you like, and you can meet one person from outside your household in an outdoor public place provided you stay two metres apart," he said at a Downing Street COVID news conference.
And ominously for the PM, on the same day the Metropolitan Police tweeted: "Have you been enjoying the hottest day of the year so far?
"It is important that we all continue to stay alert. You can relax, have a picnic, exercise or play sport, as long as you are on your own, with people you live with and just you and one other person."