Fancy a bit of chocolate? An afternoon Kit Kat with your cup of tea? A chunk of fruit and nut? Go on, you've earned it.

Except that in the future, chocoholics might have to work quite a bit harder to pay for their fix. The world could run out of affordable chocolate within 20 years as farmers abandon their crops in the global cocoa basket of West Africa, industry experts claim.

"Galaxy, Creme Eggs, every kind of £1 chocolate bar will be a thing of the past," warns London chocolatier Marc Demarquette, who believes a bar at £7, or its future equivalent, will be more like it. And Demarquette, who worked as an advisor for a recent BBC Panorama documentary on the troubled West African cocoa fields, is not alone. John Mason, executive director and founder of the Ghana-based Nature Conservation Research Council, has forecast that shortages in bulk production in Africa will have a devastating effect: "In 20 years chocolate will be like caviar. It will become so rare and so expensive that the average Joe just won't be able to afford it."

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: "With growers of premium cocoa beans already getting up to 45p per bar to look after their crops properly and fund their future, chocolate will go back to being what it used to be – a rarefied treat."

Perhaps the world will be happy to live with that. Mintel figures released last month show that all the growth in the £3.6bn chocolate market is in the premium sector, which means chocoholics may well be prepared to dig ever deeper into their pockets for their fix.

"We are currently selling a 70g bar for £7 – and the price will go up, as there is ever more demand for properly cultivated beans," says Demarquette.

"Of course," he adds, "there is all the difference in the world betweendecent chocolate and confectionery that is so full of sugar and palm oilthat it doesn't deserve to be called chocolate at all."

Sara Jayne Stanes, chair of the UK Academy of Chocolate, believes foodies will save the chocolate industry from extinction by paying whatever it takes for the good stuff: "I do not believe we will run out of cocoa beans, as sustainability is something that affects us all," she says.

"Over the past 10-15 years, growing curiosity and interest in the fine-chocolate end of the market has created an understanding of how it is different from chocolate confectionery," she says. Consumers must appreciate that "fine chocolate, like fine wine, will cost considerably more, as cocoa farmers stop leaving the land in search of better-paid jobs in the cities. The result will be more careful cultivation of the crops, and a greater supply of fine cocoas."

A spokesman from Cadburys doesn't deny the shortage of cheaper cocoa, but suggests scarcity might be averted through Fair Trade initiatives.

"Together with other manufacturers and the wider cocoa industry, we have been working on a number of agricultural initiatives to both increase and improve yields," he says. "Our move into Fair Trade was a separate step, to both pay a better price to farmers, and to encourage the next generation of cocoa farmers to stay within the industry."

The crisis may well be averted in Ghana, Cadbury's supply heartland and the world's second largest producer, according to Divine Chocolate, a Ghanaian manufacturer that is 45 per cent owned by a cooperative of 45,000 cocoa farmers. "The Fair Trade system helps ensure that the value of farming is delivered directly to the farmers and their communities," says its managing director Sophi Tranchell.

"The best route for sustainability is for farmers to organise themselves into larger units, to be able to manage their own farming improvements through improved remuneration, and to put them in a position where they have more influence in the cocoa supply chain. Why else should they continue?" She believes Divine Chocolate has found the right recipe: "Fairtrade – and particularly the Divine ownership model – delivers sustainability into the hands of the farmers, not the hands of the global buyers."

But it is in the Ivory Coast, by far the world's largest source of cocoa, where the future of the crop is much more uncertain. "Fair Trade doesn't really exist here," says Ange Aboa, a reporter based in the country's largest city, Abidjan, who specialises in covering the industry. "Young people are moving away from cocoa into rubber, whose price is more stable. And on top of that we have cocoa diseases like swollen shoot and black pod, which have caused a 10 per cent drop in production."

The biggest hope, he says, is a Nestlé project to replant 10m trees over the next decade: "But these are only for the cooperatives with whom they work, and the replanting will make up for about a quarter of the trees which have been lost. Their goal is to buy only from the cooperatives in future, and not top up by buying from local exporters".

This should result in better quality beans, he says, but the question of whether there will be enough of them to continue to perpetuate the world view of chocolate as a cheap energy-fix is much more questionable.

"It's hard to imagine a world without a demand for chocolate, but whether it remains the low-cost snack food it is now may well change in time," says Earthwatch's Dietsch. "If the demand for biofuels pushes up the price of the oil-palm crop it may well supplant cocoa – unless measures are taken for those farmers who still grow it to remain in cocoa production."