Malaria could be developing resistance to the most effective type of drug, according to the initial results of studies of patients in western Cambodia.
Scientists say tests indicate that treatments based on artemisinin are becoming less effective at combating the disease.
Currently, artemisinin treatments are the most effective way of fighting multi-drug resistant strains of malaria available. They usually clear the disease from the blood within three days.
However, the new studies have shown that they are taking longer to do so than before, which, researchers say, is an early warning sign that the parasite is developing resistance.
Scientists believe that the availability of fake malaria treatments in Cambodia, often containing small amounts of the real drug, is thought to be helping nurture resistance.
Professor Nick Day, of the Mahidol-Oxford Tropical Medicine Research Unit, who has been involved in the research, cautioned that the results could be devastating.
"Twice in the past, South East Asia has made a gift, unwittingly, of drug resistant parasites to the rest of the world, in particular to Africa," the Telegraph quoted the BBC as saying.
"That's the problem ... if the same thing happens again, the spread of a resistant parasite from Asia to Africa, that will have devastating consequences for malaria control," she added.