Nadhim Zahavi said he was being “obviously defamed” after questions were reportedly raised about his personal finances ahead of his appointment as chancellor.
The former Education and Vaccines Secretary, who is vying to succeed Boris Johnson, told Sky News he has “always” paid and “declared” his tax in the UK.
If he becomes prime minister, he will publish his reports annually, he promised.
According to The Observer newspaper, officials raised the “flag” before Mr. Zahavi became chancellor on Tuesday.
The 55-year-old replaces Rishi Sunak, who resigned earlier in the evening and suggested in his resignation letter that Mr Johnson’s government was failing “correct, competent and serious”.
Civil servants from the Cabinet Office’s propriety and ethics group warned the prime minister about HMRC’s “flag” on Mr Zahavi ahead of his appointment, the Observer reported.
Mr Johnson went ahead with the appointment “despite potential concerns about his tax affairs”, it said.
Such questions could “prevent someone from being awarded an MBE or an OBE” and that “the idea that he could become chancellor or even prime minister is unbelievable”, a source told the newspaper.
Another report in The Independent said Mr Zahavi’s finances had been “secretly investigated” by the National Crime Agency.
The investigation did not lead to any action and there was no suspicion of wrongdoing.
Speaking to Sky News presenter Kay Burleigh, Mr Zahavi said he was unaware of the allegations.
“So I was clearly tainted. I was told that the Serious Fraud Office, the National Crime Agency and HMRC were looking for me,” he said.
“I didn’t know about that, I’ve always paid tax, I’ve declared my taxes in the UK.”
Referring to a possible investigation, Mr Zahavi added: “I will answer any questions that HMRC have for me.
“But I’ll go further: I’m committing today that if I’m prime minister, I’m going to publish my reports every year.”
Mr Zahavi, who co-founded YouGov, is said to be worth more than £100m.
He is one of nine Conservative MPs who said they are still running for party leadership.
He was born in Iraq and said he “came here at the age of 11 without knowing a word of English.”
In his initial bid for prime minister, he added: “The Conservative Party made me who I am today.
“It gave me an education, it gave my family a home and, most importantly, it gave me hope.”
He promises tax cuts and education reform if successful.
Watch Nadhim Zahavi’s full interview with Kay Burleigh at 7am on Monday