July 8, 2013, 3:22 p.m.
Not enough anti-malarial bed nets paid for by the UK are being used around the world, ministers have been warned.
The UK has funded 25 million mosquito nets since 2010 but the National Audit Office said usage among target groups, such as children, was disappointing.
The watchdog urged the UK to work with aid recipients to "change attitudes" and to ensure proper value for money.
Ministers said the increased use of bed nets was part of a multi-faceted strategy acknowledged to save lives.
There is no vaccine for malaria and the disease killed an estimated 660,000 people last year and left 250 million people seriously ill.
The World Health Organization has warned of a $2.8bn shortfall in global funding to tackle the disease, regarded as one of the biggest health and economic challenges for countries with high rates of illness.
The UK spent £252m in 2011-12 on counter-prevention measures in 18 countries, 16 of them in Africa. The budget is expected to rise to £494m in 2014-15, making the UK the third largest global donor.
The NAO's report, which drew on first-hand research in Sierra Leone, Tanzania, Nigeria and Burma, said the countries had been "well-chosen" but questioned the effectiveness of some of the spending.
There had been a 23% increase in the number of families in the four countries owning a mosquito net since 2010 but usage by target groups such as children had increased by just 11.6%.
A target for 80% of children under five to sleep under bed nets by 2015 was in danger of being missed, it added, and better monitoring was needed of how many nets were used and how long they lasted.
Mortality rates from malaria in Africa have dropped by a third in the past decade but the NAO warned this progress was at risk of stalling and there could be "a rapid resurgence of the disease".
With some nets wrongly sized and difficult to hang, persuading the most vulnerable to use them was as much of a challenge as providing them in the first place, the watchdog said.
"Bilateral programmes use proven interventions, such as providing bed nets, and funding the introduction of rapid diagnostic tests and the distribution of drugs," said its head Amyas Morse.
"But sustained improvement will crucially depend on changing the attitudes and behaviour of the populations at risk.
"It is important that the department influence the governments of countries receiving malaria aid so that their efforts to tackle the disease match its own rising spending."
Campaign group Malaria No More said the NAO's conclusions were based on "a few countries" and the issue of 12 million nets in Ghana had produced "encouraging" results with reported usage among children up sharply.
"We know what works and thanks to the simple, cost-effective means at our disposal, global malaria death rates have been cut by 26% between 2000-2010," said its director James Whiting.
International Development Secretary Justine Greening said the watchdog's report had recognised that the UK's programmes saved lives and represented good value for money.
"Less disease and healthier populations make for more stable, productive and ultimately self-sufficient developing economies," she said. "The UK is playing its part by improving access to life-saving drugs, increasing the use of bed nets and investing in developing new technologies to tackle malaria and reduce the spread of resistance."