Rishi Sunak is likely to face more criticism at Prime Ministers Questions as he resists calls to sack his party chairman Nadhim Zahawi.
Mr Sunak has ordered his ethics advisor to investigate how Mr Zahawi resolved a tax dispute when he was chancellor.
The PM has aid there are "questions that need answering" but it is unclear what he knew about Mr Zahawis dealings with HMRC when he appointed him.
Mr Zahawi maintains he behaved appropriately.
Last summer, Mr Zahawi dismissed reports of a HMRC investigation as "smears" and threatened some who intended to raise questions about his tax affairs with legal action.
This has drawn criticism not just from the opposition, but from Lord Evans, Chairman of the Committee on Standards in Public Life.
He told BBC Radio 4s PM programme that the "apparent legal attempts to suppress this story" does not "live up to the sort of standards" the public would expect.
Senior Conservative MP Caroline Nokes has called for Mr Zahawi to "stand aside until this matter is all cleared up" but the prime minister has so far stood by him saying it is "longstanding practice" for ministers to remain in their roles while under investigation.
Mr Sunak ordered the investigation into Mr Zahawis tax affairs after it was revealed he had paid a penalty to settle his tax issue with HMRC.
The prime ministers spokesperson suggested Mr Sunak was not aware last week that Mr Zahawi had paid a penalty.
That leaves questions for former Prime Minister Boris Johnson, who appointed Mr Zahawi chancellor last summer when the matter had not been resolved.
In a statement, Mr Zahawi said he welcomed the investigation and looked forward to "explaining the facts of this issue" to the No 10 ethics adviser Sir Laurie Magnus.
Labour has argued the prime minister should sack Mr Zahawi before the investigation concludes, calling it a "pathetic attempt to pass the buck".
Downing Street sources have said they want to see the inquiry concluded swiftly.