The reason for this unimaginable shortage – which has been presaged by the doubling of cocoa prices in six years to an all-time high over the past three decades – is simple.

Farmers in the countries that produce the bulk of cocoa bought by the multinationals who control the market have found the crop a bitter harvest. The minimal rewards they have historically received do not provide incentives for the time-consuming work of replanting as their trees die off – a task that usually means moving to a new area of canopied forest and waiting three to five years for a new crop to mature.

"It's hard to maintain production at high levels in a particular plot of land every time, because of pest problems that eat away at the yields and the farms need to be rejuvenated," explains Thomas Dietsch, research director of ecosystem services at the Earthwatch Organisation. "Although research into new varieties and better management methods could solve those problems, the other challenge is that cocoa is competing for agricultural space with other commodities like palm oil – which is increasingly in demand for biofuels."

Meanwhile, as the supply of the raw material diminishes, millions of new consumers in the developing world are becoming addicted to the sweet energy-fix at the end of the processing chain. "Chocolate consumption is increasing faster than cocoa production – and it's not sustainable," Tony Lass, chairman of the Cocoa Research Association, told the annual conference of Britain's Academy of Chocolate last month.

Despite price rises on the trading floor, precious little reaches the smallholders who make up 95 per cent of growers, according to Mr. Lass, a former Cadburys trader and ethical sourcing advisor who has co-authored a book on the cocoa industry.

"These smallholders earn just 80 cents a day," he says. "So there is no incentive to replant trees when they die off, and to wait up to five years for a new crop, and no younger generation around to do the replanting. The children of these African cocoa farmers, whose life expectancy is only 56, are heading for the cities rather than undertake backbreaking work for such a small reward." As harvests diminish on the Ivory Coast, by far the world's biggest cocoa producer, crops in Indonesia, the third largest producer, have been hit by a change in weather systems, forcing cocoa prices sky-high.

Demarquette, who makes chocolate for Fortnum's and has a shop in London's Fulham Road, adds that, to make matters worse, the soil in Africa's traditional cocoa fields is rapidly becoming depleted. "In Ghana and Ivory Coast the earth is dead where trees have already been harvested – there are no nutrients left in the soil," he claims. And some farmers in West Africa have turned to child labour to compensate for the manpower shortage.

"Production will have decreased within 20 years to the point where we won't see any more cheap bars in vending machines – unless they are made with carob instead of chocolate," he says. "It's because the growers in West Africa only see 2p for every £1 bar. Even if you double that, it's no incentive for the next generation – which rightly expects decent working conditions. Those young people are heading for the cities. They won't stay around just so schoolchildren and commuters can continue to get their quick fix."

The good news for consumers is that cocoa, which can only be grown in latitudes within 10 degrees of the equator, is also being produced in South America, the Caribbean and Asia.

However Demarquette says it looks doubtful that those areas will be able to satisfy increased demand, "given the speed with which consumption is growing, with new markets like India and China coming along behind and following Western tastes".

There is already an upward trend in retail prices for quality chocolate, he notes