JENNIFER LOPEZ is so furious over toy boy Casper Smart’s BIG mouth that she’s cutting his monthly allowance by a hefty 25 percent, say sources.

J.Lo went ballistic when the 26-year-old back-up dancer blabbed during an interview that Jen would be returning to “American Idol” next season as a judge. FOX TV bosses were banking on making their own big announcement about the 44-year-old singer’s return.

“Jennifer caught hell from ‘IDOL’ producers,” revealed a source. “They wanted to hold a major press conference, not have a boyfriend leak it out.”

Although Casper pleaded ignorance, the singer was beyond steamed.

“This was Jennifer’s biggest fear,” said the source. “Casper is young and doesn’t understand the business, which makes life very difficult for her. It’s beginning to dawn on her that he’s a liability.”

Added an insider: “Casper’s slip was very amateurish. Jennifer couldn’t believe he’d be so stupid. The ‘Idol’ team had a big launch planned and he ruined it.” To make matters even more embarrassing, J.Lo had stalled on signing her contract until the studio gave Casper a cushy job on the show. Casper is begging Jennifer for forgiveness, but his sugar mama is hitting her big-spending boyfriend where it hurts most – in his wallet.

“Jennifer cut a quarter of the $10,000 monthly allowance she gives him,” revealed the insider. A rep for Lopez previously denied that Lopez provides Smart an allowance.

Friends are confounded that J.Lo. doesn’t cut Casper loose. What’s more, her mom, Guadalupe, “doesn’t approve of Casper,” said a source, and was “livid to learn they were thinking about adopting a kid, strengthening their bond.”

Still, J.Lo has it bad for Casper. Even though she was married to Latino heartthrob Marc Anthony and had romances with Ben Affleck, P. Diddy and Bradley Cooper, a source revealed: “J.Lo says Casper is the best sex she’s ever had.”