Reform UK deputy leader Richard Tice said he is "always happy to put things right" if he hasn't paid the right amount of tax.
It comes after he reportedly failed to pay nearly £100,000 in corporation tax.
Mr Tice did not pay any tax on the profits he made from four shell companies between 2020 and 2022, The Sunday Times has reported.
The companies were set up to receive dividends from his property investment firm and pass the money to their parent company, the newspaper said.
It said Tisun Investments Ltd then transferred £1,113,000 to Reform UK between March 2020 and May 2022.
In a statement on X, Mr Tice said his "long career with multiple businesses" meant his accounting was "bound to feature some errors".
He said: "In a highly successful career spanning 40 years, I have done business in 12 countries across three continents, and been a director of more than 150 companies.
"I have helped build thousands of homes, creating thousands of jobs and generating hundreds of millions of value for shareholders and investors along with many tens of millions of tax for HMRC.
"I am very proud of this record. Throughout this career I have taken professional tax advice and have always paid everything that I was advised to pay.
"Here's the reality: tax efficiency is a basic corporate responsibility and duty to shareholders. A long career with multiple businesses is bound to feature some errors.
"Naturally, I am always happy to put things right and if numbers need rechecking, of course, I will pay what is owed - be that more or less."
Dan Neidle, founder of Tax Policy Associates, wrote on X that "around £98k of corporation tax is due", plus "about £27k of interest".
Reform's Treasury spokesperson Robert Jenrick said on Sunday morning that Mr Tice "thinks he may have overpaid tax because he paid it through his personal taxation, rather than through the company".
He told the BBC's Sunday With Laura Kuenssberg programme: "If it transpires that he's underpaid tax, of course he'll settle it. But that is not his position. He thinks he's paid the right tax, and that's absolutely right.
"As far as I know, HMRC are not investigating. So there is no story."
Reform UK last week said any oversight on Mr Tice's part was "a minor administrative error".
The Boston and Skegness MP received at least £91,000 because his property investment company, Quidnet REIT Limited, did not pay the required 20% tax on the dividends before they were issued to him and his offshore trust in Jersey, The Sunday Times reported.
Mr Tice said on X that "overall HMRC received the correct amount of tax due" and that any issue was due to "complex tax technicality around dividends to certain shareholder classes in REITs".
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Labour Party chair Anna Turley said: "Richard Tice's credibility is in tatters and Nigel Farage needs to urgently explain why he remains Reform's deputy leader.
"This is a major scandal that's not going away. Tice has called for others to resign over tax errors that involved less money than this."
(c) Sky News 2026: Reform's Richard Tice responds to reported failure to pay £100,000 tax
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