Why are Jamie Foxx and Katie Holmes still hiding their relationship?

Worst kept secret in Hollywood?

Whispers have been swirling – quite vigorously – that Jamie Foxx and Katie Holmes are not only an item, but have been dating for years on the DL and are now secretly engaged!

Katie Holmes and Jamie Foxx. Photo: Getty

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So why haven’t the lovebirds gone public with their reported romance?

It’s apparently because of Tom Cruise….

Jamie and Katie's ex hubby appeared in movie Collateral together, but according to HollywoodLife.com, the former friends grew apart due to Katie’s reported involvement with the 48-year-old Django Unchained star - and they're not sure how Tom, 53, will react if they wave their relationship in his face!

Jamie and Tom appeared in Collateral together. Photo: Getty

“Jamie loves Katie so much and he prides himself on keeping his relationship with her private,” a source tells HollywoodLife.com.

“That said, if and when he does go there and put a ring on her finger, he’s not going to do it publicly. The last thing he wants is Tom going all ‘Collateral’ on him. Jamie doesn’t know Tom’s crazy side and he likes to keep it that way.”

Katie and Tom are parents to their daughter Suri, nine. Photo: Getty

In fact, we hear Katie, 37, filled in her new beau about her ex’s alleged stubborn ways.

“He’s heard war stories from Katie about what it’s like to be his bad side and he doesn’t want that drama,” the insider added.

“A friend of Jamie’s joked and told him he better be careful because Tom could morph into that crazy and demented Vincent from their ‘Collateral’ movie.

Katie Holmes spotted wearing a ring on fourth finger recently. Photo: Splash

“Jamie laughed but his friend told him it’s no laughing matter.”

Katie stepped out last week wearing a band on her ring finger, leading to speculation she's engaged to the Jamie.