Robbie Williams has told friend he believes a top gangsta rapper wants to beat him up.

According to the Mirror, the peculiar claim arose during a feud the 32-year-old singer has been having with Rod Stewart's stepson Ashley Hamilton over ownership of a song.

Stressed-out Robbie appeared to have lost the plot entirely when he told mates that one of singer/songwriter Ashley's associates had threatened to "set 50 Cent" on him.

The dispute centres on Ashley's claims that the pair worked together on the track She's Madonna on the singer's new album, Rudebox.

Ashley - whose mum Alana was Rod's first wife - alleges that Robbie eventually took the track to the Pet Shop Boys to finish and he was left without any credit or money.

The Mirror says a source close to Ashley said: "Robbie's behaviour was erratic to say the least. He was rambling and incoherent. The conversations just weren't going anywhere. He was outraged that Ashley had spoken publicly about this. He seemed to be losing the plot."

Ashley's spokesman said: "I'm aware of the calls but don't have any comment to make."

Rob's people said: "No comment."

Source: The Wharf