Low-quality and fake anti-malarial drugs flooding into markets in Asia and Africa are driving drug resistance and threatening gains made in the fight against the disease in the past decade, according to a study by global health experts.

The study found around 36% of anti-malarial drugs analysed in southeast Asia were fake, while a third of samples in sub-Saharan Africa failed chemical testing because they contained either too much or not enough active ingredient.

The researchers said the problem might be even bigger.

The emergence of resistance to artemisinin drugs - currently the most effective treatment for malaria - along the Thailand-Cambodia border - has already been documented.

"Despite a dramatic rise in reports of poor-quality anti- malarial drugs over the past decade, the issue is much greater than it seems," Gaurvika Nayyar, of the Fogarty International Center at the U.S. National Institutes of Health, wrote in a study in the Lancet Infectious Diseases journal.

"Most cases are probably unreported, reported to the wrong agencies, or kept confidential by pharmaceutical companies."

More than 3 billion people worldwide are at risk of malaria, a mosquito-borne parasitic disease which kills around 650,000 people a year, most of them babies and children in Africa.

Nayyar said many of the deaths caused by the disease could be avoided "if drugs available to patients were efficacious, high quality, and used correctly."

The World Health Organisation (WHO) estimates that while less than 1% of medicines available in developed countries are likely to be counterfeit, globally, the figure is around 10%.

The United Nations agency estimates that as much as a third of all medicines in some developing countries is fake.

As well as putting patients at risk, counterfeit drugs are a constant bane for companies like GlaxoSmithkline, Sanofi and other international drugmakers.

Nayyar's team analysed data from both published and unpublished studies that looked at chemical analyses and the packaging of malaria medicines in sub-Saharan Africa and southeast Asia where the malaria risk is highest.

Data from seven countries in southeast Asia - including from analysis of 1,437 samples of seven different malaria drugs - showed that more than a third of them failed chemical testing, nearly half were wrongly packaged, and about a third were bogus.

Analysis of data from 21 countries in sub-Saharan Africa including more than 2,500 drug samples, showed similar results, with more than a third failing chemical testing and around one fifth turning out to be fake.