The FBI is investigating possible insider trading involving billionaire investor Carl Icahn, American golfer Phil Mickelson and Las Vegas gambler William Walters, according to reports.

Investigators are looking into whether Mr Mickelson and Mr Walters may have traded illegally on private information provided by Mr Icahn about his investments in public corporations.

Mr Icahn, a legendary activist investor, told Reuters news agency that he was unaware of any investigation and said that his firm always followed the law.

He acknowledged a business relationship with Mr Walters but said that he did not know Mr Mickelson personally.

"I am very proud of my 50-year unblemished record and have never given out insider information," he said.              

Mr Walters and Mr Mickelson play golf together, the source familiar with the investigation told Reuters.

Mr Walters did not respond to requests for comment.

Spokespeople for Mr Mickelson and the FBI declined to comment.

The Wall Street Journal cited Glenn Cohen, Mr Mickelson's lawyer, as saying the golfing legend was not a target of the federal probe.

None of the three men have been accused of any wrongdoing.