In combative exchanges during PMQs on Wednesday, the Labour leader accused Sunak of being too “weak” to sack Tory chairman Nadhim Zahawi.
Zahawi is under an ethics investigation over whether he broke ministerial rules over an estimated £4.8 million tax bill he settled with HMRC while serving as chancellor.
He has refused demands he resign from his current job and Sunak has ignored calls for him to fire his cabinet ally.
A week ago, Sunak told MPs that Zahawi had “already addressed the matter in full”.
But Downing Street later revealed the PM had not been aware that Zahawi had paid a penalty to HMRC as part of the settlement when he appointed him Tory party chair.
Starmer told PMQs it was “fairly obvious” that “someone who seeks to avoid tax can’t also be in charge of tax”.
“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,” he said.
“Is he starting to wonder if this job is just too big for him?”
The comment was widely interpreted as a dig at Sunak’s 5ft 7 height as well as his ability to the the job.
Sunak dismissed Starmer’s demands he fire Zahawi, arguing the right thing to do was allow the independent inquiry he ordered conduct its investigation.
“The issues in question occurred before I was prime minister,” Sunak said, noting Zahawi settled his tax dispute while Boris Johnson was in No.10.
“Of course the politically expedient thing to do would be for me to have said this matter must be resolved by Wednesday at noon, but I believe in proper due process,” he said.
The prime minister argued he did not want to “prejudge the outcome” of the inquiry but bit it was right “we fully investigate this matter and establish all the facts”.