Malaria, which kills up to 2.5 million people a year, is developing resistance to some of the traditional treatments used against it.

Resistance to choloroquine, the main anti-malaria drug, is now widespread across Africa and is beginning to develop against pyrimethamine-sulphadoxine, its successor.

The two drugs are commonly used because they are cheap, costing less than US$0.20 per adult treatment course.

Other drugs are about 10 times more expensive, putting them out of the reach of most developing countries.

Genetic make-up

Writing in The Lancet, Professor Nicholas White and colleagues, say doctors should urgently consider using a combination of two drugs to ensure malaria is defeated.

They say that when only one drug is used, it is "relatively easy" for the malaria parasite to develop resistance since it can rapidly change its genetic make-up.

But they believe that resistance is far less likely if two unrelated drugs are used.

They estimate that a parasite resistant to two drugs at the same time would only develop once a century.

The scientists recommend that choloroquine should be given with drugs like artemisinin, although they say there may be adverse effects from taking the two simultaneously.

And they suggest that the drugs be combined in a single blister pack to make them easier to take.

Cost

Combination therapy could double the cost of treatment, they state.

But they add: "Increased short-term costs should result in overall savings in the longer term."

They say that, without immediate action, "a health calamity looms within the next few years".

For this reason, there should be no delay for more research to be conducted.