Boris Johnson "recognises the public's anger and concern" after fresh allegations of Number 10 lockdown parties emerged, prompting Scotland Yard to launch an investigation.
Metropolitan Police Commissioner Dame Cressida Dick said on Tuesday officers were now investigating potential offences over two years after being passed information from the Sue Gray inquiry.
The move plunges the Prime Minister into deeper jeopardy ahead of the publication of the investigation by Ms Gray, a senior civil servant in the Cabinet Office.
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Dame Cressida told the London Assembly’s Police and Crime Committee that investigations are carried out into “the most serious and flagrant type of breach” where individuals knew they were committing an offence or “ought to have known”.
She said “several other events” that appeared to have taken place in Downing Street and Whitehall had also been assessed, but they were not thought to have reached the threshold for criminal investigation.
Responding to the investigation, the PM's spokesman said: "The PM fully acknowledges the public’s anger and concern about what has been reported, he has taken responsibility for his judgments made and it is right the Met should be now given the time and space to undertake their investigations."
Cabinet Office minister Michael Ellis, replying to an urgent question in the Commons, told MPs: “This is a matter for the police, and the House will understand that I’m not in a position to comment on the nature or content of the police investigation.
“I have previously made clear from this despatch box that the Government recognises and I recognise the public anxiety and indignation that it appears as though the people who have been setting the rules may not have been following the rules, and I’d like to repeat that sentiment today.”
The number of reported Downing Street parties now totals 19 after it was revealed on Monday a birthday celebration was held for Mr Johnson during the first lockdown.
Downing Street conceded staff “gathered briefly” in the Cabinet Room following a meeting, after it was alleged 30 people attended and shared cake despite social mixing indoors being banned.
Labour deputy leader Angela Rayner said: "With Boris Johnson's Downing Street now under police investigation, how on earth can he think he can stay on as Prime Minister?
"Millions of people are struggling to pay the bills, but Boris Johnson and his Government are too wrapped up in scandal to do anything about it.
"Boris Johnson is a national distraction. Conservative MPs should stop propping him up and he should finally do the decent thing and resign."
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