Zahawi, who was also briefly finance minister, said on Saturday that he had settled an outstanding tax bill, adding that the authorities had described his failure to pay the sum as "careless and not deliberate".

A tax campaigner has estimated that Zahawi should have paid 3.7 million pounds ($4.59 million) tax, linked to the sale of shares in a company that he founded.

"Integrity and accountability is really important to me, including in this case. There are questions that need answering," Sunak told reporters on Monday.

"I have asked our independent advisor to get to the bottom of everything - to investigate the matter fully and establish all the facts and provide advice to me on Nadhim Zahawi's compliance with the Ministerial Code."

Sunak added that Zahawi, whose allies earlier said he would not stand down over the dispute, had agreed to fully cooperate with the investigation.

(Reporting by Kylie MacLellan, writing by Muvija M)