Hopes of controlling malaria in Africa could be wrecked by criminals who are circulating counterfeit and substandard drugs, threatening millions of lives, scientists are warning.

They are calling for public health authorities to take urgent action to preserve the efficacy of the anti-malarials now being used in the worst-hit areas of the continent.

There has been growing hope of an end to the age-old scourge of malaria, with UN and donor countries having contributed to a massive effort to get modern technological tools to endemic areas, including insecticide-impregnated bed nets and new drugs that had their origins in China, derived from artemisinin plants. Artemisinins have replaced old medicines such as chloroquine and mefloquine in places where the malaria parasite has developed resistance. It is the common fate of anti-malarials, which is why the World Health Organisation has urged that the artemisinins should only be used in combination with other drugs, which delays resistance developing.

But large parts of Africa are threatened by the distribution of fake and poor quality anti-malarials made illicitly in China.

Malaria kills nearly a million people each year, mainly young children and pregnant women. It is caused by parasites injected into the bloodstream by mosquitoes.

Some of the fake drugs contain artemisinin, but not enough to kill all the parasites in a child's body. Not only will the child struggle to recover, but the parasites that survive may become resistant to the drug and spread a form of the disease that ACTs (artemisinin combination therapy) will no longer cure.

In a study in the Malaria Journal, Dr Paul Newton from the Wellcome Trust-Mahosot Hospital-Oxford University Tropical Medicine Research Collaboration in Laos and a team of colleagues report on the make-up of some of the fake anti-malarials on sale in Africa, as well as some that are equally useless and dangerous because they are of poor quality. They looked at samples of suspect drugs from 11 countries collected between 2002 and 2010.

Analysis showed some counterfeits contained a mixture of wrong active pharmaceutical ingredients, some of which may initially alleviate malaria symptoms but would not cure malaria. Worse still, these unexpected ingredients could cause potentially serious side effects, particularly if they were to interact with other medication that the patient was taking, such as anti-retroviral therapies for HIV.

The fake drugs appear to have been made in eastern Asia. The researchers identified pollen from that region in some of the tablets. In 2001, police in Guangzhou, China, arrested Nigerian and Chinese men for production of counterfeits of the anti-malarial halofantrine. There was no evidence from the pollen analysis to suggest any of the drugs would have been manufactured in Africa, but production facilities for packaging materials for counterfeit anti-malarials have been seized in Nigeria.

It is impossible to say how widely counterfeit and substandard drugs are being distributed, but, Newton said something needed to be done. "The enormous investment in the development, evaluation and deployment of anti-malarials is wasted if the medicines that patients actually take are, due to criminality or carelessness, of poor quality and do not cure," he said.

"Malaria can be readily treated with the right drugs of good quality, but poor quality medicines, as well as increasing mortality and morbidity, risk exacerbating the economic and social impact of malaria on societies that are already poor."

Newton and and his team have called urgent action from public health authorities to prevent the circulation of the fake and ineffective drugs and also to stop artemisinin being used on its own, which makes it far more likely that resistance will develop than if it is in combination.

It will be very hard for the affected African countries to tackle the problem, however.

"It is very difficult to regulate the drug supply in poor countries," he said. "WHO has said that 30% of drug regulatory authorities don't function. They don't list which they are but logically they are likely to be in economically poor, malarious countries. It is very difficult for them to control the import or introduction of artemisinin therapies."