A government watchdog group, Public Citizen, said on Wednesday it has asked lawmakers to investigate whether Carl Icahn violated lobbying disclosure laws, a complaint the billionaire investor denied and called a "witch hunt."

The group said Icahn may have been acting as a lobbyist when he advised President Donald Trump to overhaul the U.S. biofuels program. Icahn, an unpaid adviser to Trump on regulation, submitted a proposal to Trump last month to change the U.S. Renewable Fuel Standard by shifting the burden of blending biofuels into gasoline away from oil refining companies, and further down the supply chain to marketers.

Public Citizen said that, because Icahn owns a controlling stake in a refinery that could benefit from the proposed change, he may have been required by a 1995 lobbying disclosure law to disclose his discussions with Trump on the subject as lobbying. The group said it has made its request for a probe in a letter sent to Congress on Wednesday morning.

"All of this has occurred with no record of any (Lobbying Disclosure Act) filings by or on behalf of Mr. Icahn," Public Citizen said in a copy of the letter provided to Reuters.

Icahn did not respond to Reuters requests for comment but described the complaint in an opinion piece on The Hill website as a "gross misstatement of the facts" and said he had vetted his activities with lawyers.