KATIE HOLMES is in a forbidden ro­mance with a close pal of ex-hubby Tom Cruise, and she’s desperate to keep it secret, says a source.

The stunning single mom, 34, has been hot for Oscar winner Jamie Foxx, 45, for months, and he regularly spends the night at Katie’s New York home, the source adds.

Tom – who co-starred with Foxx in the 2004 thriller “Col­lateral” – apparently has no clue, and Katie is said to be de­termined to keep it that way.

“Neither Katie nor Jamie want this affair out in the open,” re­vealed an insider, who’s familiar with intimate de­tails of the fling.

“Jamie wants to keep it quiet because he’s credited Tom with launching his movie career when they co-starred in ‘Collat­eral,’ and Tom would consider it a betrayal of their friendship.

“As for Katie, she thinks Tom would hit the roof if he knew and might try to punish her through their daughter Suri!”

Katie and Jamie dismissed reports they were dating after being spotted holding hands and dancing at the Apollo in the Hamptons benefit in East Hampton, N.Y., on Aug. 24. And Katie’s reps recently told The ENQUIRER that “they are friends, nothing more.” Jamie’s reps didn’t respond when we asked for comment on Oct. 11, but a day later he was photo­graphed out and about with a girlfriend and his young daugh­ter Annalise.

“It looks like Jamie’s attempt to cover up his relationship with Katie,” said a source.

Meanwhile, sources tell The ENQUIRER that Katie’s romance with Jamie has heated up while he films the “Annie” remake around the Big Apple.

“Katie and Jamie are incredibly smit­ten with each other,” said the insider. “She says he’s an intelligent, sen­sitive guy with a smoking hot body. Besides that, he treats Katie like a lady and really lis­tens to her.

“They talk on the phone a lot, and he’s stayed overnight at Katie’s apartment in Chelsea numerous times.”

 While their busy schedules pre­vent them from being together 24/7, the two are constantly in contact, said the source.

Insiders say Katie’s new romance will devastate Tom, who considers Jamie an honorary uncle to 7-year-old Suri.

The two actors became close after Tom pushed Jamie to be cast as his co-star in the 2004 thrill­er “Collateral.” Along with that break, Jamie also credited Tom for helping him win a Best Actor Oscar for playing music legend Ray Charles in “Ray.”

As The ENQUIRER has reported, Ka­tie’s love life had been floundering since her divorce. She flipped for actor Luke Kirby when they filmed “Mania Days” ear­lier this year but then discovered he was still seeing his former girlfriend, Andrea Sarubbi.

Katie’s finally rebounded with Jamie, who also has a love child named Corinne, a 19-year-old cheerleader at USC. But sources say the former “Dawson’s Creek” cutie is determined to keep the relationship on the down low.

“This is a very private and special affair, and neither Katie nor Jamie is ready to go public with it,” said the source.

“I’m sure they accept it’s only a matter of time before they’re found out, but until then, they’re having fun and taking things one day at a time.”

Meanwhile, Tom’s reaction could be explosive, said the source. “I can’t imagine how Tom will feel about his so-called friend, a guy he helped career-wise, romancing his ex-wife,” said the source.  “There’s bound to be fireworks!”