Boris Johnson is facing the most politically-perilous moment of his premiership as his involvement in a series of No 10 drinks parties held in the midst of England’s coronavirus lockdowns remains the subject of a Metropolitan Police investigation and in the wake of five of his most senior aides tendering their resignations.

       Whitehall mandarin Sue Gray has now delivered her long-delayed report on “Partygate” to the prime minister and a version of it has been released, albeit heavily-redacted at the request of the Met, still carrying out its own inquiry into 12 such events and with 300 photographs and 500 documents in its possession courtesy of Ms Gray.

       Ms Gray’s 12-page “update” on the scandal blasted “failures of leadership and judgement” in Downing Street and the Cabinet Office, describing the behaviour of some personnel as “difficult to justify”.

       The report considered a total of 16 separate social events at the PM’s Westminster residence and other government departments that took place while Covid regulations imposed strict limits on gatherings anywhere in the UK.

       In a scathing comment on the culture at No 10 under Mr Johnson’s leadership, the senior civil servant wrote: “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”.

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       Her report also revealed that Mr Johnson’s birthday celebration is among the dozen gatherings being investigated by Scotland Yard, as is an alleged party in the prime minister’s private flat.

       Addressing the House of Commons on Monday afternoon, the PM said he “accepts Sue Gray’s general findings in full” and “above all her recommendation that we must learn from these events and act now”.

       He said he was “sorry for the things we simply didn’t get right and also sorry for the way that this matter has been handled”.

       “I get it, and I will fix it,” he added. “I want to say to the people of this country I know what the issue is. It is whether this government can be trusted to deliver, and I say ‘yes we can be trusted to deliver’.”

       But during the course of his desperate bid to defend himself on Monday, Mr Johnson falsely accused his opposite number, Sir Keir Starmer, of being responsible for the failure to prosecute paedophile Jimmy Savile during his tenure as director of public prosecutions (DPP) between 2008 and 2013.

       An official report at the time made clear that Sir Keir had played no part in decisions that prevented the prosecution of the prolific sex offender prior to his death in 2011 but Mr Starmer did issue an apology in his capacity as DPP on behalf of the Crown Prosecution Service.

       The outcry over the smear has rumbled on all week, finally forcing Mr Johnson to issue a grovelling mea culpa on Wednesday.

       But that was not enough for his long-loyal adviser and director of policy Munira Mirza, who sent him a resignation letter on Thursday saying the “scurrilous” Savile jibe was an “inappropriate and partisan reference to a horrendous case of child sex abuse” and told Mr Johnson: “You have let yourself down.”

       Her exit was swiftly followed by three more, as the PM’s communications chief Jack Doyle, chief of staff Dan Rosenfield and principal private secretary Martin Reynolds all likewise jumped ship – with Downing Street hurriedly briefing that their exits were all part of a pre-planned shakeup of Mr Johnson’s inner circle.

       A day later, Elena Narozanski, another member of the Downing Street policy unit, became the fifth aide to join the exodus.

       Chancellor Rishi Sunak has meanwhile condemned the Savile remark, a gesture that is being interpreted as a clear attempt to draw a line between himself and the ailing PM, who continues to live in fear of Sir Graham Brady announcing that the 1922 Committee of Tory backbenchers has received the 54 letters of disapproval needed from MPs to trigger a no-confidence vote.

       A leadership challenge from Mr Sunak is being strongly touted.

       Having initially said in December that he was “sickened” at the prospect of Downing Street employees ignoring social restrictions at events reported to have taken place in May, November and December 2020, Mr Johnson found himself sidestepping questions about whether he too had attended an event on 20 May after an explosive leaked email provided evidence that over 100 staff were invited to attend the bash and “bring your own booze”.

       No 10 stonewalled questions over that party – pointing to Ms Gray’s then-active investigation – but anger only grew among Tory MPs and Conservative-leaning newspapers.

       Finally, at Prime Minister’s Questions (PMQs) in the House of Commons at high noon on Wednesday 12 January, Mr Johnson did address the issue, confirming that he had attended the garden gathering for around 25 minutes with the intention of thanking his staff for their efforts during the pandemic while claiming, somewhat improbably: “I believed implicitly that it was a work event.”

       He apologised, expressed empathy for the public fury in light of the personal sacrifices millions had made and again implored his critics to await the outcome of Ms Gray’s inquiry before passing judgement.

       That cut little ice with his opposition number, Sir Keir describing the prime minister as a “pathetic spectacle of a man who has run out of road” and “without shame”, derided his apology as “worthless” following “months of deceit and deception” and called on him to resign, the seething disdain in his voice drawing chuckles of laughter from across Parliament while members of Mr Johnson’s frontbench sat stoney-faced behind their Covid masks.

       “Why does he think the rules do not apply to him?,” Sir Keir asked, incredulous, voicing the thoughts of millions.

       Both Liberal Democrat leader Sir Ed Davey and Ian Blackford, the Scottish National Party leader in the Commons, in turn called on Mr Johnson to “do the decent thing and resign”.

       Support for the embattled prime minister subsequently trickled in from Cabinet colleagues (and potential leadership challengers) like Mr Sunak, deputy PM Dominic Raab and foreign secretary Liz Truss, without a great deal of enthusiasm evident.

       Since then, news of more parties has emerged to add to the total, including two separate events on 16 April 2021, the evening before Prince Philip’s funeral when the Queen sat alone in accordance with strict Covid rules as she bade farewell to her husband of 73 years.

       Both events are said to have been leaving parties for staff working in the prime minister’s inner team, with wild tales told of “excessive alcohol” being drunk, attendees dancing to music DJ’d by a special adviser beyond midnight and a staffer being sent out to the local branch of Co-op to fill a suitcase with bottles of wine.

       As further details were leaked to the newspapers in dribs and drabs, Mr Johnson invited further exasperated ridicule when he told Sky News during a hospital visit on Tuesday 18 January: “I can’t believe we would have gone ahead with an event that people said was against the rules… Nobody warned me it was against the rules, I am categorical about that – I would have remembered that.”

       After news of a sixteenth and final party, the aforementioned gathering to celebrate Mr Johnson’s birthday, emerged, the Met announced its investigation, prompting the delay of Ms Gray’s report as Scotland Yard requested that sensitive details pertaining to its own areas of inquiry be removed.

       Prior to those final outrageous twists in the saga, a poll found that two-thirds of the public (66 per cent) believed the PM should resign over his role in the parties.

       But will Boris Johnson go? The calls for him to step aside have been deafening.

       Douglas Ross, the Scottish Conservative leader, warned: “If he [the PM] has breached his own guidance, if he has not been truthful, then it is an extremely important issue... If the prime minister has misled Parliament, then he must resign.”

       Despite Tory MP Jacob Rees-Mogg’s astonishing dismissal of Mr Ross as “a lightweight figure” during an interview with the BBC’s Newsnight, his comments stand as a damning indictment of Mr Johnson’s wayward leadership.

       Subsequent condemnation from Mr Johnson’s predecessor, Theresa May, and other senior Tories like Andrew Mitchell in the wake of Ms Gray’s report being made public have only ramped up the pressure.

       But the PM and his apologists like culture minister Nadine Dorries have continued to reach for the increasingly farcical defence that he cannot comment on the allegations until all inquiries have concluded – shifting their emphasis from the civil servant to the investigating officers.

       The reaction of Tory backbenchers to all of this will be crucial in determining whether he can “ride out” the scandal or be forced to face the music.

       Before that heated PMQs on 12 January, backbench Conservative MP Nigel Mills said his position would be “untenable” and it would be a resigning matter if he was found to have been in attendance.

       Clearly not. Any frequent observer of the Johnson administration will point to this PM’s reluctance to sack his own ministers for breaches of the ministerial code, with Matt Hancock’s departure as health secretary last summer a rare exception.

       There are many different paths towards a potential resignation for Mr Johnson, but just two years after winning a thumping majority at a general election, he is unlikely to resign on his own accord.

       As Conservative commentators have previously highlighted, the party has a brutal record of deposing of leaders it no longer sees an electoral asset and a forced exit is the most likely route for any resignation.

       Either the Cabinet could launch a full-scale revolt with senior ministers telling the PM to stand down (perhaps unlikely) or 15 per cent of Conservative MPs could submit their letters to Sir Graham.

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       The number of letters submitted at any given point is a closely guarded secret but, if the required number is reached, it would trigger a vote of confidence in Mr Johnson and his premiership would be on the line. Ms May, whose premiership was dealt a terminal blow by her ill-fated election gamble, survived a confidence vote in December 2018 but eventually resigned six months later.

       For the time being, however, Mr Johnson’s allies are adamant he is “going nowhere”, with a minister insisting: “The prime minister retains the confidence of the people of this country and he did so two years ago with the biggest majority in decades”.