Too few of the millions of anti-malaria bed nets bought for African nations by UK aid are actually being used, a spending watchdog suggested, calling into question the value for money for taxpayers.
Britain - among the world's leading nations in the global fight against the killer disease - has supplied nearly 25 million mosquito-proof nets since 2010 and expects the number to reach 45 million by 2015.
But the National Audit Office said that despite net ownership increasing by almost a quarter of households in some badly-affected nations, their use among the most vulnerable groups had increased by only 11.6%.
Department for International Development (DfID)-led information campaigns to encourage more people of the benefits "had not yet resulted in sufficient behaviour change", it found
And excluding usage figures from official DfID performance data made it harder to judge whether UK anti-malaria spending - expected to hit almost £500 million in 2014/15 - was good value.
The watchdog also criticised a failure to secure sufficient improvements in the domestic health systems of developing countries, without which UK cash might not result in a "sustained impact".
A decade of sustained global funding has succeeding in reducing malaria mortality rates in Africa by a third - a million lives - with the UK set to become the third most generous contributor after the US and the multilateral Global Fund.
But international development minister Lynne Featherstone warned recently that push could "go to waste" amid signs investment is levelling off - telling affected countries they also needed to do more.
Globally malaria kills 655,000 each year and makes 250 million severely ill and is estimated to stunt economic growth in the most severely affected countries by 1.3% a year.
The stated aim of the UK work is to halve the number of malaria deaths in at least 10 high-burden countries by 2015.