The creator of Celebrity Big Brother has accused Five of "ripping off" its format for the new fly-on-the-wall programme.
Five's Back To Reality features 12 celebrity contestants who have all appeared on previous reality shows, including Big Brother faces Jade Goody, Nasty Nick, and Craig Phillips.
Five has described Back to Reality as the "ultimate reality show" as it openly borrows aspects from every major reality programme.
But Endemol, which makes Big Brother, has written to Five's lawyers after identifying similarities between the shows.
The letter warns that it is keeping an eye on Five's show, although it does not go as far as launching legal action.
A spokeswoman for Endemol in the Netherlands said: "There has been a letter sent from our side to Five to object against Back to Reality, which is in our view a rip-off of Celebrity Big Brother.
"We are now considering our position."
A Five spokesman defended the show, saying: "We are confident Back to Reality does not infringe anyone else's format rights."
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