The German motor industry's proud reputation for reliability suffered a serious setback yesterday when research revealed that its prestigious makes were increasingly likely to suffer mechanical failure.
Models built by Audi, BMW, Mercedes-Benz and Volkswagen are now more likely to let down their owners than Fords and most Japanese models, a study by Which? magazine found.
Authors of the biggest consumer study of its kind said reliability was suffering on some German models as manufacturers extended their range and sophisticated electrics suffered teething troubles.
Feedback from the drivers of almost 33,000 cars up to eight years old found that those least likely to need repairs were Japanese. Five of the top seven names in the "best" category were Japanese: Honda, Nissan, Mazda, Toyota and Lexus, a Toyota marque.
For the first time, using the current methodology, a German car manufacturer has fallen into the lowest category. VW, which uses the advertising slogan "If only everything in life was as reliable as a Volkswagen", was previously rated "average" but has fallen into the "poor" category this year. The reliability of mainstay models such as the Golf, Polo and Passat may have suffered as VW diversified into more prestigious brands such as the £68,000 Phaeton limousine and the £50,000 Touareg off-roader, Which? said.
Mercedes Benz, the most reliable European brand in the past two years, dropped the furthest of German car makers in the past year, according to Which? readers, whose responses saw the make demoted two categories from "best" to "average". Cars produced since 2001 have let it down with poor scores for breakdowns, faults and minor problems, the report said.
Audi cars were also downgraded from "good" to "average", the category also occupied by BMW, which still has a poor breakdown rate.
During the same period Ford cars have risen in the estimation of their owners from "average" in terms of reliability to "good", especially for the Focus, Fiesta and Mondeo.
Research suggested that popular models were becoming less reliable as manufacturers extended their range of makes or were involved in takeovers.
Richard Headland, senior researcher at the Consumers' Association, said: "The big German manufacturers are still average, but the trend for them is going the wrong way. Among various factors are consolidation when a company takes on new brands and globalisation, which means that parts are sourced from all over the world."
The Automobile Association's Motoring Trust said car reliability had improved dramatically since 20 years ago when serious problems such as engine failure and seized gearboxes were more common.
John Stubbs, head of technical policy, said: "The problems tend to be smaller and less critical, and as consumers we are more ready to complain."
Helen Parker, editor of Which?, said: "Brands are built on a promise, for example that they will deliver consistently a certain level of quality or value. If customers see through the promise or find it's been broken, it can be the worst possible news for a brand. Our survey reveals the first robust evidence of bad news for some of the biggest car brands in the world."