Keir Starmer has said Tories are deserting "rat" Boris Johnson and compared the remaining Cabinet ministers to the "charge of the lightweight brigade".
Mr Starmer turned the screws on the embattled Prime Minister, as he battles to save his leadership in the face of a wave of resignations.
The Labour leader told Tory MPs to "have some self respect" and abandon Mr Johnson, as he condemned the PM for his handling of the Chris Pincher saga.
MPs fells silent as Mr Starmer read the words of a man who alleged he had been sexually assaulted by the former Deputy Chief Whip, who quit last week amid claims he groped two men.
But the shameless Prime Minister said he would "keep going" as he had a "colossal mandate" - despite two Tory MPs telling him to his face to quit.
The PMQs clash came after an explosive 24 hours, which saw Cabinet Ministers Sajid Javid and Rishi Sunak resign their posts, triggering a slew of departures from Tory ministers and aides.
Mr Johnson faced laughs and jeers as he kicked off PMQs by joking "Today's a big day", referring to changes to National Insurance Contributions coming into force.
But moments later Health Minister Jo Churchill became the eighth minister to quit, posting her resignation letter on Twitter as the PM got to his feet.
Mr Starmer began his questions by reading the testimony of a man who accused Mr Pincher of assault to a grim silence from MPs.
He said: "I accept that is not easy listening, but it is a reminder to all those propping up this Prime Minister just how serious this situation is.
"He knew the accused minister had previously committed predatory behaviour but he promoted him to a position of power anyway. Why?"
Mr Johnson hit back, saying Mr Pincher had lost his Government job and the Conservative whip.
The Prime Minister added: "I want to say to him that I abhor bullying and abuse of power anywhere in Parliament, in this party or in any other party."
But he dodged a question on whether he joked "Pincher by name, pincher by nature" - adding: "I'm not going to trivialise what happened."
Mr Johnson admitted that he should not have appointed Mr Pincher to the role of Deputy Chief Whip but claimed an allegation made against Mr Pincher in 2019 when he was Foreign Office Minister had been resolved.
Mr Starmer asked him: "No denial, and he says the matter was resolved when he means it was upheld.
"They're all sitting there as if this is normal behaviour. When that young man reported his attack to a Government whip, she asked him if he was gay, when he said that he was, she replied 'that doesn't make it straightforward'."
He went on: "Will he apologise for those disgraceful comments on behalf of this Government?"
The Prime Minister replied: "I have already said that I regret very much that the member for Tamworth continued to hold office after the complaint was made against him in the Foreign Office, and it was resolved in the Foreign Office, his apology was accepted, but clearly that was not enough. And, in hindsight, I should have realised that he would not change."
However, Mr Johnson insisted that when he was given the information that the Labour leader read out about the complaint that was made he "acted immediately".
Mr Pincher has denied multiple allegations against him going back years.
The Labour leader continued to attack the PM over his handling of the saga, saying: "Awful behaviour, unacceptable in any walk of life, it's there for all to see but he ignores it.
"It was the same when his ally was on the take from the lobbyists. It was the same when his home secretary was bullying staff. It was the same when taxpayers' money was being abused and it was the same when he and his mates partied their way through lockdown.
"Anyone quitting now after defending all that hasn't got a shred of integrity. Isn't this the first recorded case of the sinking ships fleeing the rats?"
The floundering PM hit back at Mr Starmer by accusing him of opposing Brexit and trying to get Jeremy Corbyn elected to No10.
But Mr Starmer retorted: "What a pathetic spectacle, the dying act of his career is to parrot that nonsense."
He added: "As for those who are left, only in office because no-one else is prepared to debase themselves any longer. The charge of the lightweight brigade. Have some self-respect."
Mr Johnson had a tough time from some of his own MPs but boasted about his “colossal mandate” and vowed to “keep going”.
In a blunt clash, Tory rebel Tim Loughton told him: "Does the Prime Minister think there are any circumstances in which he should resign?"
The PM replied: “Clearly, If there were circumstances in which I felt it was impossible for the government to go on and discharge the mandate we’ve been given, or if I felt for instance we were being frustrated in our desire to support the Ukrainian people, or over some major point, then I would.
“But frankly the job of a Prime Minister in difficult circumstances, when he’s been handed a colossal mandate, is to keep going.
“And that’s what I’m going to do!”
