A tiny change could mean the malaria parasite becomes resistant to the newest drug weapon against the disease, researchers have claimed.
Mosquitoes carry the malaria parasite
Artemisinins were hailed because they succeed where other drugs failed.
But the St George's University of London team found one protein component can control whether the parasite is sensitive or resistant to artemisinins.
However, in Nature Structural and Molecular Biology they say the finding could help them develop other drugs.
Malaria kills over one million people a year.
The team found that artemisinins work by interfering with the action of a protein called PfATP6 which should regulate the amount of calcium present in the malaria parasite's cells.
The cells need these calcium transporters to drive their molecular motors.
Artemisinins stop the transporter doing its job, which means that the level of calcium grows because there is no regulation - and the cell dies.
However, the team found changing just one component - or amino acid - in the protein means the artemisinin has no effect.
The team now have to carry out further research to see if the same effect occurs when they examine the whole parasite, rather than just the transporter.
Professor Sanjeev Krishna, who led the research, said: "This is an incredibly worrying observation."
He added: "We are really lucky not to have seen resistance to artemisinins in malaria parasites so far.
"History of other antimalarials tells us that resistance is highly likely to develop when artemisinins are used widely.
"At least we know one more thing we should now be watching out for in parasites. We also urgently need to find new drugs."
He said that as soon as signs of artemisinin resistance were seen, scientists should act to restrict the spread of the strain as quickly as possible, potentially by using other drugs as well as artemisinins.
"You have to clamp down on it as soon as possible".
But he said the discovery could also be seen as good news.
"It might give us a chance to see what changes we could make to get around this problem.
"Before we had this knowledge, we were working in the dark. Now at least there is a glimmer of light."
Andrew Read, professor of natural history and an expert in malaria research at the University of Edinburgh, said: "It's very disappointing that it's a single amino acid change that can give rise to resistance.
"That's something that's very easy for evolution to do out in the wild.
"But we probably shouldn't be surprised. Malaria parasites have done very well at evolving around other drugs we've thrown at it."