The charity Oxfam has cast doubt on an international scheme that aims to boost the provision of the most effective treatment for malaria.
The UK government has contributed £70m to the Affordable Medicines Facility for malaria (AMFm).
Oxfam says there is no evidence the programme has saved the lives of the most vulnerable people.
The body behind the AMFm says an independent study shows it has improved access and reduced drug prices.
The scheme was introduced three years ago by the Global Fund to Fight Aids, TB and Malaria.
It acts as a global subsidy to provide greater access to combination therapy for malaria, particularly through private-sector drug retailers in developing countries.
The idea is to reduce the use of older treatments that carry a higher risk of resistance, and to untap the potential of the private sector in reaching remote communities.
More than 200 million people contract malaria every year and 655,000 die from the disease - most of them are young children.
The scheme is being piloted in seven countries including Kenya, Ghana and Nigeria. Its future will be considered at a meeting of the Global Fund's board next month.
Oxfam has criticised it as "risky and dangerous".
The charity's senior health policy advisor, Dr Mohga Kamal Yanni, said: "It is dangerous to put the lives of sick children in the hands of a shopkeeper with no medical training, and to pursue a scheme that doesn't help those people who need it the most.
"There is no cheap option or short cut to combat malaria.
"The AMFm is a dangerous distraction from genuine solutions like investing in community health workers, who have slashed the number of malarial deaths in countries such as Zambia and Ethiopia.
"The Global Fund board must act on the evidence and put a stop to the AMFm now."
The Global Fund said Oxfam's claims were "simply untrue".
In a statement, it said: "Some Western aid groups oppose a pragmatic approach that includes any involvement of the private sector.
"But the reality of this programme is that it is getting life-saving medicine to people who need it most from the private sector outlets where they already seek treatment.
"Before the launch of AMFm, life-saving malaria treatments cost up to 20 times as much.
"An extensive study has shown that AMFm has increased availability and reduced prices for high quality anti-malarial drugs."
The UK's Department for International Development (DfID) allocated £40m to the scheme in its first two years, and boosted it by £31.6m this year. AMFm is also supported by the Canadian government and the Bill and Melinda Gates Foundation.
A DfID spokesman said: "DfID is helping to halve the number of malaria deaths in the most badly affected countries by 2015 in a number of ways, including improving access and availability of life-saving drugs.
"Studies have shown that quality drugs have got through to remote areas - and that more vulnerable groups, including children under five in rural areas and from the poorest backgrounds, are now being reached."