Drivers are being warned not to rely on personal breathalysers this Christmas.

As part of its annual drink and drug-driving campaign, Dorset Police, along with its partners, said that tests carried out by Dorset County Council’s Trading Standards team had found that those relying on shop-bought breathalysers would put both themselves and other road users at risk.

Out of 14 devices tested, nine indicated that the user was still safe to drive when the legal drink drive limit was actually exceeded.

Officers also found false claims of links to the police on four of the devices, which were bought over the internet.

Inspector Matt Butler, of Dorset Police’s Traffic Unit, said: “This survey revealed that home breathalyser kits can be unreliable, so we are urging people not to use them.

“Research has shown that just one drink can impair someone’s ability to drive. Our message is very clear – if you are going to drive, don’t drink any alcohol at all.”

Ivan Hancock, Trading Standards service manager for Dorset County Council, said: “Our research shows that there is a wide variety of cheap breathalyser kits available for sale on the internet.

“Our tests have highlighted that the majority of these are unreliable and may give anyone using them a dangerously-false sense of security.

“We have also identified some being sold with descriptions that cannot be substantiated, which are likely to mislead anyone buying one.

“Drivers would be extremely foolish to rely on the readings they get from one of these cheap devices. Use your head. Just don’t drink and drive.”

This year’s drink and drug-drive campaign has received the backing of Margaret Waterland, whose husband David, 68, of Wareham, was killed by a drink-driver in Milborne St Andrew near Blandford in May 2013.

Stewart Rosoman, 51, was jailed for six years in November 2014.

Margaret said: “Drink driving is simply not worth it.

“It is not only innocent lives you destroy, but you also ruin your own family’s lives – they suffer too.”