More than a third of antimalarial drugs sold in Africa have failed quality tests, research shows.

The study of drugs bought in Ghana, Kenya, Nigeria, Rwanda, Tanzania and Uganda shows that 35 per cent contained too little active ingredient or failed to dissolve, rendering them ineffective. Another third of treatments also belonged to a class of drugs that the World Health Organisation (WHO) wants to be banned because they can cause the malaria parasite to develop resistance.

The findings suggest that hundreds of thousands of lives are being put at risk and indicate that drug counterfeiting is to blame. Researchers fear this is spreading to Africa from South-East Asia, where up to 50 per cent of anti-malarial drugs are fake.Malaria kills up to 2.7million people each year, chiefly in Africa, and counterfeit and sub-standard drugs may account for at least 200,000 deaths.

A third of the drugs bought as part of the study were artemisinin monotherapies of a sort that the WHO wants banned in favour of combination treatments. It is feared that the parasite Plasmodium falciparum may become immune to artemisinin in isolation, as it has to older drugs such as chloroquine.

Only 40 of 74 manufacturers of artimisinin drugs have agreed to stop making monotherapies, and 42 countries, including 18 in sub-Saharan Africa, still allow these drugs to be sold.

Roger Bate, of Africa Fighting Malaria, based in the US, who led the research, said: “Substandard drugs not only endanger lives today, but also jeopardise future malaria treatment by accelerating parasite resistance.”

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