Tom Cruise refuses to star alongside best friend Will Smith, because he fears he'll be upstaged - according to reports.

The pair are close pals and frequently speak of plans to act together - but sources tell the National Enquirer Cruise is terrified a collaboration would kill his career.

The insider claims Cruise backs down over every script Smith sends him and never returns the offer.

The source says, "Tom's a great guy, but he sees Will as competition - a movie like I Am Legend is the kind of role Tom used to get offered first. The feeling is that Tom fears he'll suffer by comparison if he acts alongside Will. He hasn't really worked with another strong male lead since Dustin Hoffman in Rain Man."

The source insists Smith doesn't care: "He knows how Tom is, and it's not going to end their friendship if they never appear on screen together."