Doctors say they are in a "race against time" to halt the spread of potentially untreatable malaria.
A strain of Plasmodium falciparum has emerged on the border of Thailand and Burma that is resistant to the most effective treatments. Experts fear if it cannot be defeated many thousands more lives could be lost to the disease, with the strain marching on to India and Africa.
P. falciparum is the most deadly form of the mosquito-transmitted parasitic organism, and responsible for nine out of 10 malaria deaths. It is found in tropical regions around the world but most common in Africa.
In 2010, malaria killed an estimated 655,000 people worldwide, mostly young children and pregnant women.
The strain is believed to be the same as one already established in Western Cambodia, where the first cases were reported in 2009. Now there is evidence that it is gaining a foothold 800 kilometres away along Thailand's north-western border with Burma.
The parasite has undergone a genetic change making it resistant to artemisinin, the cornerstone of frontline falciparum treatments. Artemisinin derivatives, such as artesunate, are combined with older drugs which on their own have ceased to be effective.
Since 1994, falciparum malaria on Thailand's western border has been kept in check by an artesunate-mefloquine combination.
A team of British and Thai scientists measured the effectiveness of artemisinin treatments in more than 3,000 malaria patients who attended clinics in the region. Over a 10-year period between 2001 and 2010, the average time taken to reduce the number of parasites in the blood by half rose from 2.6 hours to 3.7 hours.
This is said to be a clear sign that the drugs are becoming less effective. The proportion of slow-clearing infections increased over the same period from six to 200 out of every 1,000 cases.
The scientists, funded by the Wellcome Trust, spelled out their fears in a paper published in The Lancet medical journal.