The world's largest diamond has been denounced as a fake.

A British property developer reportedly claims it is all part of a scam and the gem is actually a lump of plastic.

Brett Jolly says he will go to police about the alleged hoax in South Africa.

He claims he is the victim of a fraud aimed at persuading him to buy the land in South Africa's North West province on which the so-called diamond was found.

The uncut stone was said to be about seven-thousand carats, but Jolly says he has been duped.