The Prime Minister was aware that the Metropolitan Police officers had begun their work but decided against telling his top team when they met on Tuesday.
Their phones surrendered before entering the Cabinet Room, ministers were unaware of the dramatic development outside when Commissioner Dame Cressida Dick had announced the investigation was underway.
Downing Street said Mr Johnson judged it was best not to tell them about the “sensitive” matter because he did not want to “pre-empt a police statement”.
Though it is not entirely clear who knew what and when, ministers are unlikely to have known about the development for a full 20 minutes as the rest of Westminster reacted.
The first sign that the Metropolitan Police chief was planning to announce the investigation into alleged Covid breaches at the heart of Government was at 9.28am, when the move was reported by the Guido Fawkes website.
The Cabinet meeting began two minutes later, but ministers would have given up their phones as in normal protocol.
It would not have been until 10.30am when the meeting finished that Cabinet members began receiving back their phones to learn the news announced at around 10.11am.
During a briefing with journalists, the Prime Minister’s official spokesman confirmed Mr Johnson had been made aware of the investigation before the meeting began.
He said Mr Johnson “didn’t reference it specifically” but only “alluded to it” by insisting the saga would not deter the Government from “getting on with the job”.
Asked why Cabinet ministers were not told about the police investigation, the Prime Minister’s official spokesman said: “From what I understand it’s important not to pre-empt a police statement on this sort of issue at any point.
“That was the judgment the Prime Minister made.
“It was at that stage unclear exactly at what point the Met would make that statement and obviously the Prime Minister will continue to discuss any relevant issues with his Cabinet.”
The spokesman added: “I think it’s understandable that, given the sensitive nature of what the Met were due to announce, it’s right that wasn’t pre-empted in any way.”
Cabinet members left No 10 without answering the questions of journalists waiting in Downing Street, but Commons Leader Jacob Rees-Mogg did launch a defence of Mr Johnson, saying he was “honoured to be under his leadership”.
He told reporters in Downing Street: “The leadership of Boris Johnson this country has had has been so brilliant – that he has got us through this incredibly difficult period and he’s got all the big decisions right.”
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