An anonymous investor has bought up 7% of the world market in cocoa beans, apparently aiming to make a killing out of chocaholic consumers.

Shortage of supply has helped to double cocoa prices over the past 18 months, pushing them to a 15-year high, and it is thought the speculator - who has bought the rights to about 200,000 tonnes of future cocoa crops - is hoping to cash in by driving the price even higher.

Western countries consume about $60bn (?38bn) worth of chocolate bars every year, and with prices soaring, analysts believe a controlling interest in next year's cocoa crop could be a lucrative buy. However, the move is unlikely to push up the price of a Mars bar just yet.

Kevin Watkins of Oxfam, said yesterday there was a more serious issue at stake. "This is potentially very destabilising. The benefits of any increase in price will not reach the cocoa producers who have borne the brunt of low prices in recent years.

"This just demonstrates that something serious needs to be done about monopoly behaviour in commodities markets."

The International Cocoa Organisation - which includes most of the world's big producers, including Ghana and the Ivory Coast - is forecasting a 130,000 tonne deficit in cocoa production by the time this year's season ends in September. Stocks are expected to be 11.2% down on a year ago.

Cocoa for September delivery rose $48 a tonne on Friday to $1,799 a tonne, at the New York Board of Trade - one of the two principal markets for the commodity - together with the Liffe, where the mystery speculator has been buying up contracts