The discovery of the "world's biggest diamond" last August sent ripples of excitement around the world. It was said to be twice the size of the Cullinan, or Great Star of Africa, discovered near Pretoria in 1905.

Neither the South Africa Diamond Board, nor the Diamond and Jewellery Federation, the trade body, confirmed its authenticity. But the warnings were drowned out by the breathless claims of one Brett Jolly, a spokesman for the mining firm Two Point Five Construction, who had announced the stone was being transported to a bank vault in Johannesburg "until we calm down and decide what we are going to do".

Now however, the "diamond" has been revealed as a fake. Yesterday, the president of the World Federation of Diamond Bourses, Ernest Blom, announced he was withdrawing from the verification process to test the stone, saying: "I suspect something is afoot."

Meanwhile, as the diamond's authenticity began to unravel, Mr Jolly, originally a British property developer, claimed he had been the victim of a fraud and told the website Mining Weekly Online that he wished he "never was involved with [the diamond] in the first place," adding that he didn't "care any more whether it's a diamond or not".

The mining website claims that on Monday it was offered a chance by Mr Jolly to test the stone, on the condition that it sign a non-disclosure agreement, in which it could be sued if it disclosed any information on the stone before receiving written confirmation. Mining Weekly Online declined the offer.

Meanwhile, De Beers, which owns the Cullinan, expressed relief that it still holds the title of the biggest diamond ever discovered. "The search for diamonds is so romantic," said a spokesman, Tom Tweedy. "This does illustrate that diamonds hold a mystique with people."