Thousands of motorists are being misled when it comes to maintaining their cars’ warranties, according to an undercover investigation by What Car? magazine.

Mystery shoppers contacted 200 dealers from the 10 top-selling manufacturers in the UK. Posing as car owners they asked whether the warranty would be invalidated if the car was serviced outside of the franchised network.

BMW came out worst in What Car?’s survey.

According to What Cars? survey, seven of its 20 dealers quizzed claimed the warranty would be invalidated by getting a BMW serviced outside the main dealer network.

Next worst in the investigation was Citroen (6 out of 20) and Ford (5 out of 20). The best brand was Toyota, where one dealer made the same mistake.

Four in 10 dealers gave mystery shoppers false or confusing information about where customers could and couldn’t take their cars to be serviced.

Nearly 20% of the dealers surveyed incorrectly said a car’s warranty would definitely be invalidated if serviced outside of the franchised network, while a further 21% said it would risk and probably invalidate their warranty cover.

In the past, owners of cars still covered by the manufacturer’s warranty had to take them to the main dealer for servicing.

Now, however, as a result of a campaign by the Office of Fair Trading, car owners have the right to shop around for competitive servicing prices, and can take their cars to their local independent garage. Warranty cover will not be affected, so long as cars are serviced to manufacturer schedules and standards using manufacturer parts.

Of the dealers questioned, 36 said the warranty would definitely be invalidated and a further 43 said it would put warranty cover at risk. If a car loses its warranty, an owner is unable to claim for repairs if things go wrong in the first three years of ownership.

“Vehicle manufacturers must make clear the conditions of new car warranties to their franchised dealer networks, as many car dealers are still unsure about what will and what will not invalidate a new car warranty,” says Alan Pulham, franchised dealer director of the Retail Motor Industry Federation.

“The majority of dealers are fully aware of the conditions of new car warranties, and the survey confirms this. However, some are obviously still not getting the information they need,” says Pulham.

“It is unfortunate that we still have this level of confusion over an issue which has already been made very clear several times over.”