In a Prime Minister’s Questions held less than 24 hours after the resignations of former Cabinet ministers Rishi Sunak and Sajid Javid, the Labour leader branded the scenes “a pathetic spectacle”.
Mr Johnson refused to answer, amid direct questioning from Sir Keir, why he had originally promoted former deputy chief whip Chris Pincher.
The Prime Minister claimed he abhorred “bullying and abusive power anywhere in Parliament, in this party or any other party” as he said in “hindsight”, he should have realised Mr Pincher would have not changed.
But Sir Keir said: “What a pathetic spectacle. The dying acts of his political career is to parrot that nonsense. And 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 Speaker, for a week he's had them defending his decision to promote a sexual predator. Every day, the lines he’s forced them to take have been untrue. First that he was unaware of any allegations – untrue.
"Then, he was unaware of any specific allegation – untrue. Then he was unaware of any serious specific allegation, and now people are being sent out to say he simply forgot that his whip was a sexual predator. Anyone, with anything about them, would be long gone from his front bench.
"In the middle of a crisis doesn’t the public deserve better than a Z-list cast of nodding dogs.”
Ministers and aides have continued to submit their resignations on Wednesday morning, while support is ebbing away from the Prime Minister among previously-loyal MPs.
But Mr Johnson is understood to have told allies that he is “not going anywhere” and his critics should “calm down”.
On Wednesday morning, Robin Walker resigned as schools standards minister, telling the Prime Minister the “great achievements” of the Government have become “overshadowed by mistakes and questions about integrity”.
Will Quince quit as children and families minister, saying he could not accept being sent out to defend the Prime Minister on television with inaccurate information over the Chris Pincher row.
Treasury economic secretary John Glen quit, telling the Prime Minister he could not reconcile staying in the job with “the complete lack of confidence I have in your continuing leadership of our country”.
Victoria Atkins resigned as a minister in the Ministry of Justice, telling Mr Johnson: “I can no longer pirouette around our fractured values. We can and must do better than this.”
Laura Trott resigned as a ministerial aide, saying “trust in politics is – and must always be – of the utmost importance, but sadly in recent months this has been lost”, while Felicity Buchan also stood down as a parliamentary private secretary, calling for “fresh leadership”.
Their resignations followed a string of departures from the Government on Tuesday evening, led by Rishi Sunak and Sajid Javid, who delivered broadsides at Mr Johnson as they quit their Cabinet posts.