At a meeting of all Tory MPs on Monday, Mr Johnson attempted to reassure his colleagues, revealing that Sir Lynton Crosby, the Australian campaign guru, was to play a more central role advising him.

However, attempts to shore up party unity were cracking on Monday night as Downing Street suffered its first resignation from the Tory benches over the saga.

Angela Richardson announced she was quitting as a parliamentary private secretary to Michael Gove, citing her “deep disappointment” in Mr Johnson.

She wrote on Monday: “Sue Gray’s report published today clearly states that there were failings at No 10 that let us all down … there could have been an early acknowledgement and apology.”

Ms Gray’s long-awaited report into “partygate” claims included no specific details about what went on at the 16 events she looked at and named no individuals who attended.

But it did announce broad findings and a string of criticism about the events – some reportedly attended by more than 30 people with alcohol served – that took place during lockdowns.

“Against the backdrop of the pandemic, when the Government was asking citizens to accept far-reaching restrictions on their lives, some of the behaviour surrounding these gatherings is difficult to justify,” Ms Gray wrote.

“At least some of the gatherings in question represent a serious failure to observe not just the high standards expected of those working at the heart of Government but also of the standards expected of the entire British population at the time.”

Of the 16 events Ms Gray looked into, a dozen were now being investigated by the Metropolitan Police, Ms Gray confirmed.

They include a gathering on November 13 2020 in the Downing Street flat, where Mr Johnson now lives with his wife, Carrie Johnson, and two children.

That was the day Dominic Cummings had been forced out of Downing Street after serving as Mr Johnson’s senior adviser.

The Mail on Sunday reported that during the evening attendees in the flat played Abba songs, including The Winner Takes It All. Mr Johnson has previously denied it was a party.

At least three other events which the Prime Minister may have attended are also being investigated: a Downing Street garden drinks event, an alleged birthday party and a farewell gathering for a special adviser.