British Prime Minister Rishi Sunak Monday ordered an investigation into a wealthy ally's murky tax dealings as he again vowed "integrity" in his government after Boris Johnson's scandal-plagued tenure.

As well as the probe into Conservative party chairman Nadhim Zahawi, Sunak faced hostile questions about the appointment of BBC chairman Richard Sharp, a former banker who acted as a go-between to help funnel a loan to Johnson when he was in 10 Downing Street.

The apparent scandals surrounding well-off individuals threaten to become a political distraction for Sunak as he battles to restore the Conservatives' standing in the polls in the midst of Britain's worst cost-of-living crisis in decades.

Sunak refused to accede to opposition demands to fire Zahawi, who reportedly settled a seven-figure demand from tax authorities with a fine for late payment when he served briefly as chancellor of the exchequer last year under Johnson.

The prime minister instead commissioned a probe by his newly appointed ministerial ethics advisor, Laurie Magnus, conceding that "clearly in this case, there are questions that need answering".

Sunak appointed Zahawi as party chairman, and cabinet minister without portfolio, when he entered Downing Street in October.

He deflected questions about Zahawi's activities prior to then, as concerns mounted about whether Johnson knew of the tax investigation when he appointed the Iraqi-born politician as chancellor and head of the UK's tax authority.

Zahawi welcomed the probe and insisted he did nothing wrong in the tax case, which stems from his co-founding of the successful polling company YouGov in 2000.

But opposition parties have pointed to his shifting explanations as more details have emerged in newspaper reports, and to his prior threats of libel lawsuits against journalists and a tax consultant. (AFP)