OK Magazine reports the 64-year-old former U.S. track and field athlete first started using body-shapers to slim down for special events, but has become addicted to them.
"He started wearing them under his suits on the red carpet, and he loved the results so much that now he wears them all the time," an insider told the publication, adding that Jenner is upset his estranged wife Kris Jenner recently shared his secret with the couple's daughters Kendall and Kylie.
"The girls haven't stopped laughing, and Bruce was furious that Kris revealed something so private, especially with all the cross-dressing rumors," the source continued. "He insists the Spanx are just to hide his belly -- not an excuse to wear women's underwear."
While it's unclear whether Jenner has been using women's or men's Spanx, the reality star has been linked in the past to speculation that he wishes to have a more feminine look. Jenner was scheduled to undergo a Laryngeal Shave -- a procedure often requested by patients undergoing a sex change -- in December 2013, but decided to cancel the surgery after the news leaked to the press. Additionally, the Kardashian patriarch has been growing his hair long, despite Kris' disapproval, over the past year.
"Bruce beats himself up for not having the washboard abs he had as a pro athlete," the insider said of Jenner's Spanx-wearing habits. "He wants people to think that he's still ripped."