Hollywood heavyweight Jack Nicholson has has undergone secret gastric bypass surgery after ballooning to a deadly 360 pounds! That’s the conclusion of a top doctor — after exclusive RadarOnline.com photos revealed a telltale scar above the 80-year-old actor’s belly button! According to pals, however, despite having undergone the secret surgery Nicholson still can't stop eating!

“You can clearly see a linear scar above his belly button,” said Dr. Gabe Mirkin, who has not treated the Chinatown star. “This is a definitive sign of gastric bypass surgery. This allows food to bypass his stomach and go directly to the much smaller intestines — forcing him to eat less.”

“But he hasn’t lost much weight, judging by his size,” the doctor said. “He’s still over 350 pounds!” The three-time Academy Award winner’s belly was busting out of his shirt, and he barely fit in his chair while sitting courtside with 25-year-old son Ray. Jack’s massive bulk and bloated body indicate he’s “clearly suffering from morbid obesity,” said Dr. Mirkin. “Look at his hands! He has swelling of all three joints in his fingers. The only thing that does that is osteoarthritis, which is an arthritis caused by obesity.”

Judging by the scar’s size and condition, Dr. Mirkin concluded Nicholson went under the knife at least three weeks before he was snapped at a Los Angeles Lakers game on Oct. 19. While The Departed star’s decision to undergo surgery is encouraging, the doctor pointed out it’s far from a permanent weight loss solution! “You can’t beat obesity with just bypass surgery. Patients eventually get around it by eating more frequent, smaller meals,” Dr. Mirkin tells Radar.

The formerly handsome As Good as It Gets star — photoed here in his thinner years — was a “chronic drug user in the early years of his career, experimenting with LSD in the early 1960s and taking cocaine and marijuana,” Marc Eliot wrote in his book “Nicholson: A Biography.” Jack’s sexual appetite led to high-profile affairs, including romances with actresses Anjelica Huston, Rebecca Broussard and Lara Flynn Boyle. According to insiders, Jack picked up a lust for life from his late next-door neighbor, Marlon Brando. “Like Marlon, Jack loves everything to excess,” said a source. “People still see him smoking his cigars and sometimes he’ll start wheezing so hard that his eyes start tearing, and the fear is that it could be emphysema. Besides that, he eats every meal like it’s his last.”

The excesses have eclipsed his work — and his career has suffered. Jack’s last starring role was in 2010’s How Do You Know, and a movie he’s signed to do with Kristen Wiig, “Toni Erdmann,” is still in the development stage. “It’ll be a miracle if Jack makes it that far,” warned the insider. “Every step is a huge effort and he just looks bloated and uncomfortable.”