Billy Joel is secretly preparing for a bitter divorce battle over his $100 million fortune, say sources.

The ENQUIRER recently revealed that the Piano Man’s 27-year-old wife had started a cozy relationship with a fashion designer young enough to be the musician’s son.

"Billy is furious at the prospect of being two-timed by Katie Lee," divulged a close source."If he gets proof she’s been cheating, he’ll divorce her real fast.

"He signed Katie Lee to a prenup when they married in 2004, but he’s preparing for a nasty $100 million divorce battle!"

THE ENQUIRER recently disclosed that Joel – who turns 60 on May 9 – was advised by close pals to dump gorgeous Katie Lee when his tour ends in November. For nearly a year, she’s been spotted out with hunky Israeli-born designer  Yigal Azrouel, 36.

After The ENQUIRER ripped the lid off the comfy relationship, the close source said Katie Lee told Billy they "need to spend time together" while he takes a break from his tour.

"Katie Lee is fearful she’s driven Billy away," explained the close source. "Billy feels betrayed because he’s given her everything to keep her happy.

"When Katie Lee found his $32 million Long Island mansion ‘dull and boring,’ he purchased a cool $6 million condo in Manhattan so she could entertain her young friends. Then she wanted a beach house, and Billy paid a whopping $16.5 million for an oceanfront home.

"Billy also helped her land a gig as host of the first season of Top Chef in 2006, and he’s helping her open a restaurant.

"But pals have told him Katie Lee has been playing him for a fool. They’ve agitated him by saying, ‘This is how she’s repaying you.’

"He says, ‘If this is true, she won’t get a penny!’"