Sparks flew both on screen and off when elegant Audrey Hepburn teamed up with married leading man William Holden in the 1954 movie “Sabrina”. Their sizzling romance was destined to crash and burn – but not because Holden was already hitched!

In fact, the serial cheater was in the habit of bringing his mistresses home to meet his wife Ardis, a stunning actress whose screen name was Brenda Marshall (who starred in “The Sea Hawk” with Errol Flynn).

As the mother of Holden’s three children, Ardis typically looked the other way, confident that her straying husband would always come back to her. But Audrey had a different effect.

The cultured young beauty had captivated Holden and was a “wife’s worst nightmare,” according to author Edward Epstein, in his new book “Audrey and Bill”.

“Ardis’ heart must have frozen at the sight of her,” Epstein wrote.

“Here was this radiant creature, 15 years younger than she, who wasn’t merely beautiful but had unique qualities of class and an almost spiritual grace.

“The others Bill had brought home were beautiful, but not like this.”

As soon as Audrey left, Ardis went in “attack mode” and forbade Holden from ever seeing her again. But he was totally smitten with the young Oscar winner and ignored his wife’s ultimatum for what he would later call “the love of my life.”

Incredibly, they managed to keep their steamy affair under the radar, even after a “Sabrina” crew member accidentally caught them romping in Holden’s dressing room.

But the romance ended just weeks after Audrey’s trip to meet Ardis. The Brussels-born beauty had always dreamed of having several babies and retiring from acting to raise them. Little did she realize that Holden had already undergone a vasectomy.

“Once while chatting brightly about the names of their future children, suddenly an embarrassed smile, tinged with fear, crept into Bill’s face,” writes Epstein. “He told her that the only thing they could not have together was children.”

The revelation stunned Audrey. She couldn’t believe that he hadn’t mentioned this to her earlier. “He would recall the fixed expression in her eyes – how she stood looking at him like a hurt, bewildered child,” says the author.

With both her family dreams and trust shattered, Audrey instantly ended the illicit relationship. She embarked on a rebound romance with troubled actor Mel Ferrer. And to lay to rest any whispers of an affair with Holden, the studio arranged to have her announce her engagement to Ferrer at Holden’s house.

“Audrey’s eyes avoided Bill’s,” Epstein says of that awkward night. “His bloodshot eyes underscored his angst. He was clearly a man carrying a torch.”

The former lovebirds did reunite for the 1964 film “Paris When It Sizzles”, but by then the spark was long gone and the movie flopped. Audrey had a son with Ferrer and another boy with her second husband, psychiatrist Andrea Dotti before dying of colon cancer in 1993.

Holden went downhill due to booze problems. He killed the driver of another car in an Italian DUI crash, finally divorced Ardis in 1971 and bled to death after an inebriated fall at home in 1981.