Climate change is already wreaking havoc on the livelihoods of small-scale tea and coffee farmers in some of the world's poorest countries, according to a three-year research project by Fairtrade drinks producer Cafédirect.

Research across four countries – Kenya, Mexico, Peru and Nicaragua – carried out with the state-funded German Technical Corporation, showed that growers are already being forced uphill to higher altitudes, at a rate of three to four metres a year on average, as temperatures rise. "A huge number of growers are now experiencing increased instances of pestilence and disease from rises in temperature. They are also facing prolonged drought and changing weather patterns," said Cafédirect chief executive, Anne MacCaig.

She argued that the priority for developed countries should be helping the world's poor to protect themselves against climate change.

"What's crucial is that there's an option of sustainable adaptation to safeguard the supply chain. Climate change is affecting those least able to deal with it. We can't underestimate that."

Smaller producers, who are reliant on a single crop and often cannot afford to install costly irrigation equipment as temperatures rise, are worst affected, the project, known as AdapCC, found.

Some farmers could see their incomes fall by up to 90% in the next fifteen years, the researchers say. They argue that worldwide 30 million farmers will be affected.

Small-scale growers in Peru have seen yields fall by 40% since last year, compared to 30% across the country as a whole; small producers in Mexico have seen yields halve, against a national decline of 7%, Cafédirect says.

Tea and coffee are on the climate change front line because they only grow in a relatively narrow temperature range. Research suggested that all four of the countries involved would see the quantity and quality of their crops decline sharply over the coming years.

Cafédirect worked on different ways of helping farmers adapt. In Kenya, it helped growers diversify into new crops such as passion fruits; in Peru, farmers were able to use their land to sell carbon credits; and elsewhere they planted native tree species to help bind the soil and prevent mudslides.

The report came as the government said that it will donate £12m to the Fairtrade Foundation, with the hope of doubling the number of developing country farmers who are awarded the Fairtrade mark.

Fairtrade products pay a premium to relatively small-scale growers, helping to protect them from the vicissitudes of global commodity markets and the buying power of vast multinationals. The Fairtrade mark is celebrating its 15th birthday, and now covers a wide range of products, from bananas to chocolate. Sales of Fairtrade products were up by 43% in 2008.

Addressing a conference of Fairtrade supporters yesterday, development minister Douglas Alexander said: "Fairtrade products are already a big part of life in the UK, with new products appearing on our shelves every day. Our £12m funding will help improve this even further." The Fairtrade market as a whole is expected to treble, to £9bn, by 2013.

Harriet Lamb, executive director of the Fairtrade Foundation, said: "In the current economic climate, it's the poorest communities who are hit the hardest, and so positive business models like Fairtrade, which deliver increased development benefits from trade, are more important than ever."