A former Conservative Attorney General has said Boris Johnson should quit over the "corrosive" Partygate saga.
Jeremy Wright said he could not be sure whether the Prime Minister had lied to Parliament over the extent of lockdown boozing in Downing Street but "real and lasting" damage had been done.
In a 2,300-word statement on his website, which appeared to have been taken down and then republished, Mr Wright argued that the PM had been "negligent" in failing to prevent rule-breaking events and should quit.
His intervention comes as Boris Johnson scrambles to shore up his authority in the wake of Sue Gray's scathing Partygate report.
The top civil servant said Mr Johnson must bear responsibility for the raucous partying in No10, detailing examples of vomiting, fighting and aides abusing security staff and cleaners.
The PM said he had been "humbled" by Ms Gray's findings but claimed he was unaware of the brazen antics of some staff.
But his apology has failed to quell anger among some Tories, with a fresh slew of MPs breaking cover to call for him to quit.
In a statement, Mr Wright said: "I also find it inconceivable that senior officials and advisers would have tolerated, facilitated and even encouraged the breaking of Covid rules if they believed that the Prime Minister would have been horrified and outraged by what was happening in Downing Street when he was not there."
Mr Wright, who was the Government's top legal adviser for four years, said he feared "real and lasting damage" had been done by the scandal and would hinder the Government's efforts to deliver for the public.
He said: "I have therefore, with regret, concluded that, for the good of this and future Governments, the Prime Minister should resign."
The statement appeared on his official website, but the web link later directed to "page not found''. It later reappeared.
Mr Wright said the PM's former Principle Private Secretary Martin Reynolds "comes off worst" in Sue Gray's report and should have been responsible for making sure No10 staff acted as the PM wished.
In a sign that staff knew what they were doing was dodgy, the former top aide - who was dubbed "Party Marty" - said "we appear to have got away with [it]" after a BYOB bash in the Downing Street garden in May 2020 which he organised.
Mr Wright said: "If leadership is in part about setting the right tone for the organisation you lead, the tone represented by the routine disregard for the spirit, and often the letter of the Covid rules which Sue Gray describes betrayed at best a casual and at worst a contemptuous attitude to the sacrifices made and distress felt by the many who observed rigorously both spirit and letter of those rules.
"I find it impossible to accept that the Prime Minister does not bear some personal responsibility for that tone.
"Frankly, the fact that leaving events of the kind described by Sue Gray may have been within the rules does not make them wise in the context of the example which should have been set in Downing Street, and the Prime Minister should have found other ways to thank departing members of staff, as other managers around the country had to do."
Senior Tory backbencher Tobias Ellwood warned that the party would lose the next election unless they ousted the Prime Minister.
Mr Ellwood, an arch critic of Mr Johnson, told Sky News: "We still seem to be in denial. It's time to shake off this partisan Stockholm syndrome, I believe.
"Our party brand is suffering. We will lose the next election on the current trajectory as reflected in recent elections.
"There is not only just a concern on the conduct of behaviour in No10, because that has breached the trust with the British people, it is now concerns about No10 thinking what our policies are."
It comes as No10 refused to deny that another party took place in Downing Street on the Prime Minister's birthday in June 2020.
Mr Johnson was fined by the Metropolitan Police over a surprise birthday gathering in the Cabinet Room that day.
It has now been reported that the PM's wife Carrie - who was also fined for the do - hosted a separate gathering with several friends in the Johnson's No11 flat later that day.
Asked about the Sunday Times report, a No 10 spokesman said: "I have seen the same reporting that you have but I think this is covered in the terms of reference in Sue Gray's report where she clearly said that any other gatherings ... where she received credible allegations, would be looked into.
"Downing Street staff were given clear guidance to retain any relevant information and co-operate fully with the investigation."
Mr Johnson's popularity among the Tory grassroots has plummeted after being briefly bolstered by the response to the war in Ukraine.
The latest league table by Tory bible ConservativeHome gave him the lowest score of any Cabinet member on -15 net approval.
The site said that the "driver of his low scores is that the Government is too left-wing, at least in the view of many activists".
