Keir Starmer has ridiculed "hopelessly weak" Rishi Sunak for failing to sack the Tory Chairman Nadhim Zahawi amid a row over his tax affairs.
Mocking the 5ft 7in Prime Minister, the Labour leader asked him whether is starting to wonder whether the role in No10 "is too big for him".
Mr Sunak suggested the "politically expedient" option would have been to sack Mr Zahawi, who failed to appear at PMQs, but added: "I believe in proper due process."
Mr Starmer replied: "His failure to sack him when the whole country can see what's going on shows how hopelessly weak he is.
"A Prime Minister overseeing chaos, overwhelmed at every turn. Is he starting to wonder whether this job is just too big for him?."
The SNP Westminster leader Stephen Flynn added the issue of Mr Zahawi's tax affairs "is now a matter of the Prime Minister's own integrity and accountability".
Just last week Mr Sunak stood by the Tory Chairman and told MPs that Mr Zahawi had "already addressed this matter in full".
But on Monday he was forced to order his ethics adviser Sir Laurie Magnus to investigate the Cabinet minister amid a row over the party chair's tax affairs.
In a bruising Prime Minister's Questions today, Mr Sunak tried to pass the buck by saying the issue occurred before he was appointed Prime Minister.
"No issues were raised with me when he was appointed to his role and since I commented on this matter last week more information has come forward and that is why I have asked the independent adviser to look into the matter.
"I obviously can't pre-judge the outcome of that".
The PM also accused Labour of "simple political opportunism" for urging him to appoint an ethics advisor then wanting a decision before the adviser had investigated the case.
No10 was later unable to say that Mr Sunak is confident no more damaging surprises will emerge about Mr Zahawi's tax affairs.
The Prime Minister's press secretary said: "I don't think any of us can predict what may come up but I think he's confident now he's put the right process in place and the independent adviser is looking into it."
Downing Street also indicated Mr Sunak received assurances about Mr Zahawi's tax affairs before defending him at Prime Minister's questions last Wednesday.
The Tory chairman said at the weekend a "careless but not deliberate" error had been made and he had agreed to a settlement to fix it, estimated to be around £5million, including a penalty.
But some Tories, including ex-minister Caroline Nokes, have called on Mr Zahawi to stand down as party chair while the probe is carried out.
Earlier on Monday the ex-Tory Cabinet Minister David Gauke said the PM should “encourage” him to resign ahead of what will be a “very uncomfortable” PMQs.
The former MP said it was “hard to see” how the row “doesn’t ultimately end” in him stepping down as Tory chairman.
