The widely used insect repellent Deet appears to be losing its effectiveness against mosquitoes, scientists say.
Researchers from the London School of Hygiene and Tropical Medicine say mosquitoes are first deterred by the substance, but then later ignore it.
They say more research is needed to find alternatives to Deet, which was first developed by the US military.
The research was carried out on Aedes aegypti, a species of mosquito that spreads dengue and yellow fever.
The findings are published in the journal Plos One
Dr James Logan, said: "The more we can understand about how repellents work and how mosquitoes detect them, the better we can work out ways to get around the problem when they do become resistant to repellents."
Deet - or N,N-diethyl-meta-toluamide - is one of the most widely used active ingredients in insect repellents. It was developed by the US military, following its experience of jungle warfare during World War II.
For many years, it was not clear exactly how the chemical worked, but recent research suggests that insects simply do not like the smell.