Golfer Phil Mickelson wagered more than $1 billion on different sports over the past three decades, according to an upcoming book by a renowned professional gambler who formed a "betting partnership" with the six-time major champion.

In an excerpt from "Gambler: Secrets from a Life at Risk," Billy Walters writes that he first started teaming up with Mickelson in 2008, using the golfer's access to high-stakes offshore bookmakers to make larger bets than he could otherwise.

According to the excerpt released Thursday on FirePitCollective.com, Walters says it was not uncommon for Mickelson to "bet $20,000 a game on long-shot, five team NBA parlays. Or wager $100,00 or $200,000 a game on football, basketball and baseball."

These are some of the biggest revelations from the memoir:

Mickelson racked up close to '$100 million' in gambling losses

Using his own personal records and additional information from other sources, Walters estimates Mickelson's gambling losses were far greater than the $40 million that has has been previously reported. "(M)uch closer to $100 million," Walters writes. "In all, he wagered a total of more than $1 billion" over the past three decades.

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

Ryder Cup near-miss

Walters says Mickelson "liked to gamble as much as anyone I’ve ever met," and related one example from the 2012 Ryder Cup in which the American squad – featuring Tiger Woods, Bubba Watson and Lefty himself – was a heavy favorite over Europe.

"He was so confident that he asked me to place a $400,000 wager for him on the U.S. team to win," Walters writes. "I could not believe what I was hearing."

Likening the situation to Pete Rose betting on baseball, Walters refused to make the bet. "You’re seen as a modern-day Arnold Palmer," he says he told Mickelson. "You’d risk all that for this? I want no part of it."

Walters didn't know if Mickelson was able to place the bet elsewhere, but at what came to be known as the "Miracle at Medinah," the Europeans erased a 10-6 deficit on the final day to claim the Ryder Cup – with Mickelson's loss to Justin Rose a major factor in the Americans' loss.

What did Phil Mickelson say?

After a nine-hole pro-am Thursday, Mickelson was asked for comment on the allegations in the excerpt and declined, saying, “I’m gonna pass today. We’ll talk later.”

Later in the evening, Mickelson posted the following statement on social media:

"I never bet on the Ryder Cup. While it is well known that I always enjoy a friendly wager on the course,  I would never undermine the integrity of the game. I have also been very open about my gambling addiction. 

"I have previously conveyed my remorse, took responsibility, have gotten help, have been fully committed to therapy that has positively impacted me and I feel good about where I am now."

Betting partnership turns sour

The relationship with Mickelson ended in 2014 as Walters became caught up in an insider trading scandal.

Walters says he could have been spared a prison sentence if his former friend was only willing to tell a "simple truth" he had previously shared with the FBI.

"I never told him I had inside information about stocks and he knows it," Walters writes. "All Phil had to do was publicly say it. He refused."

Walters, now 76, was convicted on insider trading charges in 2017 and sentenced to serve five years in prison, but was released in 2020 to finish the rest of his term at home due to the coronavirus pandemic.

His new book is set to be released August 22.

Contributing: Adam Woodard, Golfweek