Some insect repellents which use electronic buzzers to deter mosquitoes are ineffective, a consumer group says.
Mosquitoes spread diseases such as malaria
Holiday Which? said the devices did not stop mosquito bites during user tests and should be withdrawn from sale.
It has passed its findings to Trading Standards, warning people who use the repellents may be given a false sense of security.
Manufacturers said while nothing gave 100% protection, their products were effective and based on sound science.
The electronic buzzers emit high or low frequency sounds - designed to replicate the wing beats of either a male mosquito, or a dragonfly, which feeds on the insects.
The theory is that this should frighten female mosquitoes - which can carry diseases such as malaria - away.
Holiday Which? expressed particular concern about a device marketed by the company Prince Lionheart, which performed badly in tests against two species of mosquito.
The device is designed to clip on to babies' cots to protect infants from attack by mosquitoes.
In total 18 products were tested - each six times - by a team at the London School of Hygiene and Tropical Medicine, with 30 mosquitoes used in every trial.
Buzzers all poor
All four buzzer devices tested produced disappointing results - but other products did much better.
Holiday Which? editor Lorna Cowan said: "Some repellents offered complete protection and zapped mozzies in next to no time.
"People are at their most vulnerable when they're asleep so plug-ins are a worthwhile investment, though in malarial areas people should also use impregnated bed nets.
"We think others, such as the buzzers we tried, were a shocking waste of money, didn't offer any protection at all and offer nothing but a false sense of security. They should be removed from sale."
Repellents containing plant oils were less effective than their synthetic counterparts, the researchers found.
Four types of plug-in device which emitted mosquito repellent vapours were 100% effective when tested.
Holiday Which? said the best repellents for use outdoors contain at least 30% diethyl tolumide, known as Deet.
Masta and Lifesystems wrist and ankle bands soaked in a Deet solution gave excellent although not total protection.
Anthony McCarron, of Prince Lionheart UK Ltd, said the "avoidance technology" used in their Electronic Mosquito Repeller was solid and had been proven in field tests.
But he said no mosquito repellent was 100% effective against all types of mosquito in all situations.
He also added that the Holiday Which? tests were not necessarily realistic, as the mosquitoes had been starved for several days before use.
"In environments were the mosquito is not confined or starved, such as the Which test, a mosquito is more likely to avoid predators such as the dragonfly and because of this we have found the avoidance technology to be successful."