Rishi Sunak defended his determination to motorboat an morals enquiry into Nadhim Zahawi alternatively than sacking him, astatine Prime Minister's Questions.
Sir Keir Starmer said the PM was "hopelessly weak" for not firing the curate for "seeking to debar tax".
"Is helium starting to wonderment if this occupation is conscionable excessively large for him?" asked the Labour leader.
Mr Sunak said it was Sir Keir that was anemic due to the fact that "he has nary principles conscionable petty politics".
Labour has said Mr Sunak indispensable accidental whether helium has ever paid a punishment to Revenue and Customs, similar Mr Zahawi. Downing Street has truthful acold declined to say, calling an individual's taxation arrangements "confidential".
In the House of Commons, Mr Sunak said it would person been "politically expedient" to sack Mr Zahawi arsenic a curate earlier PMQs got nether mode astatine noon but helium believed successful "proper owed process".
That was why, helium said, helium had asked inquire his morals advisor to analyse whether the Conservative Party president had breached ministerial rules.
Mr Zahawi was chancellor astatine the clip the estimated £4.8m colony was agreed with HMRC.
'Political opportunism'
Sir Keir asked Mr Sunak wherefore helium had said astatine past week's PMQs that Mr Zahawi had "addressed this substance successful full".
"Since I commented connected this substance past week much information, including a connection from by the curate without portfolio [Mr Zahawi], has entered the nationalist domain which is wherefore it's close that we bash found the facts," the premier curate said.
He accused Sir Keir of "simple governmental opportunism" for urging him to name an morals advisor past wanting a determination earlier they had investigated the case.
And helium claimed "the quality betwixt him [Sir Keir] and maine is I basal by my values and my principles adjacent erstwhile it is difficult," accusing the Labour person of indulging successful "petty politics".
'Tax avoiders'
Sir Keir said the PM's "failure" to sack Mr Zahawi showed "how hopelessly anemic helium is - a premier curate overseeing chaos, overwhelmed astatine each turn".
"He can't accidental erstwhile ambulances volition get to bosom onslaught victims again. He can't accidental erstwhile the prisons strategy volition support streets harmless again. He can't adjacent woody with taxation avoiders successful his ain cabinet," said the Labour leader.
"Is helium starting to wonderment if this occupation is conscionable excessively large for him?"
Conservative MP Nigel Mills told BBC Radio 4's The World astatine One that Mr Zahawi indispensable explicate wherefore helium had to wage a punishment to the taxation authorities.
"I conscionable don't spot however an probe into the ministerial codification resolves this due to the fact that if helium is cleared by that, that won't halt radical asking questions astir what connected world happened."
'Careless'
He added: "I deliberation the lone mode to resoluteness this is to marque wide what the concern was that gave emergence to a important penalty.
"If that tin beryllium explained we tin each determination on. If it can't, past intelligibly his presumption won't beryllium tenable."
Mr Zahawi confirmed connected Saturday that helium had made a outgo to settee a quality with HM Revenue & Customs (HMRC), adding that the taxation authorization accepted the mistake was "careless and not deliberate".
The BBC understands the quality was resolved betwixt July and September past year, erstwhile Mr Zahawi was chancellor nether Boris Johnson, and that the full magnitude paid is successful the portion of astir £5m, including a penalty.
The taxation was related to a shareholding successful YouGov, the polling institution helium co-founded successful 2000 earlier helium became an MP.
Mr Zahawi has not confirmed however overmuch his punishment amounted to, nor the full worth of the last colony with HMRC.