• Star golfer Phil Mickelson bet more than $1 billion over three decades, and sustained gambling losses of close to $100 million, Billy Walters says in a new book.
• Walters, who was convicted of insider trading in 2017, says he had a sports gambling partnership with Mickelson that spanned five years.
• He wrote that Mickelson in 2012 asked Walters to place a $400,000 bet for Mickelson on him and the U.S. Ryder Cup team to win the competition at the Medinah Country Club.
• Former President Donald Trump commuted Walters' five-year prison sentence on his last day in the White House.

Phil Mickelson wagered a total of more than $1 billion over three decades, and sustained gambling losses of close to $100 million, the golf superstar's former friend Billy Walters claims in a new book.

Walters, who was convicted of insider trading in 2017, in the book "Gambler: Secrets from a Life at Risk," detailed what he called a sports "gambling partnership" with Mickelson "that lasted five years."

He says that led to Mickelson, in September 2012, asking Walters to place a $400,000 bet on Mickelson and the rest of the U.S. Ryder Cup team to win the competition at the Medinah Country Club, where the golfer phoned from.

" 'Have you lost your f---ing mind?' "Walters told Mickelson, according to an excerpt of the book published by Golf Digest.

Walters writes that he asked Mickelson if he remembered what happened to Pete Rose, who was banned from baseball for life for betting on the Cincinnati Reds while managing that team. " 'You're seen as a modern-day Arnold Palmer,' " Walters writes he told Mickelson. "You'd risk all that for this? I want no part of it.'' '

Walters, 77, said that Mickelson replied: "Alright, alright."

Walters wrote that he hoped Mickelson did not place that bet somewhere else, as the American team lost in a stunning comeback by the European squad that became known as the "Miracle at Medinah."

The gambler says that in the period from 2010 through 2014, his detailed records and other records provided to him by sources show that Mickelson wagered $110,000 to win $100,000 more than 1,100 times, and more than 850 times had bet $220,000 to win $200,000.

"In 2011 alone, he made 3,154 bets — an average of nearly nine per day," Walters wrote.

"Based on our relationship and what I've since learned from others, Phil's gambling losses approached not $40 million as has been previously reported, but much closer to $100 million. In all, he wagered a total of more than $1 billion during the past three decades," Walters wrote.

"The only other person I know who surpassed that kind of volume is me."

CNBC has reached out to Mickelson via his online media contact site for comment on Walters' allegations.

In a 2022 interview with Sports Illustrated, the 53-year-old Mickelson said that, "My gambling got to a point of being reckless and embarrassing. I had to address it. And I've been addressing it for a number of years. And for hundreds of hours of therapy."

He also said in that interview. "I feel good where I'm at there. My family and I are, and have been, financially secure for some time."

A New York federal court jury found Walters guilty in April 2017 of 10 criminal counts related to trading on insider information with stock in Dean Foods Co., which is no longer in business. He was sentenced later that year to five years in prison.

Walters' sentence was commuted by then-President Donald Trump on his last day in the White House in Jan. 20, 2021.

The White House, in announcing that action, said Walters' commutation was supported by "numerous professional golfers, among them Mickelson.