The French ruling is not as harsh as one in Belgium last year which makes it impossible for him to advertise his cleaner to the Belgians as bagless - its main selling point.
The French court accused Mr Dyson's company of exaggerating the benefits of baglessness and unpleasantly exaggerating the defects of competitors. A TV advertising campaign for the Dual Cyclone in France last year showed a close-up of the filth, allegedly ignored by the conventional cleaners, and claimed contents were "pollen, germs, beetle excrement, animal fur".
The French appeal court said the dust used in the Dyson ads was "abnormal" and his attacks on the opposition were excessive and abusive. A similar case, brought by Electrolux against Dyson in Britain, was thrown out this month.
The French ruling may become an important test case on the rights and wrongs of negative, or comparative, advertising in Europe. A European Union directive allows such advertising from April but it remains restricted, or non-existent, in several EU countries, including France.
Mr Dyson has refused to take defeat lying down, buying full-page advertisements in French newspapers, using heroic terms to describe his 20-year battle to establish the virtues of his "revolutionary" cleaner, against the packed legal ranks of the competition.
The man who captured half the British market (in value terms) in six years, fears a campaign by other vacuum-cleaner manufacturers to bar him from continental Europe.
"I want to show consumers how powerful companies try to block their smaller competitors, while at the same time, they copy our technology and our style," he said. "I am sickened by these manoeuvres." Electrolux, the Swedish-based, European market leader, brought the French court case, along with Electrolux Filter, its bag-making subsidiary. The Swedish company says the Dyson ads were "unethical" and Electrolux is considering a case against the new ads.
Mr Dyson's Dual Cyclone is not a vacuum cleaner. It creates a centrifugal sucking action, like a salad-drying machine. It does not need a replaceable bag but has a container which must be emptied.
In adverts, Mr Dyson claims all conventional cleaners become less thorough when a new bag is fitted. Dust, he says, blocks access holes and reduces sucking efficiency. His own cleaner remains constantly efficient, he says, however full it becomes. The French court decided some claims made for the Dual Cyclone were reasonable. But Dyson overstated the case with its attacks on its rivals. Firstly, it used dust which was "not in conformity with (household) norms." Secondly, it failed to mention that what the consumer saved on replacement bags would be spent on new filters for the Dyson machine. Thirdly, Dyson had compared his pounds 250 model to the low-range Electrolux pounds 60 model.
The court stated: "The operating method of the Dyson company does not simply consist of extolling the merits of of its technical invention but extends to denigratory, aggressive and untruthful compar- isons, expressed in a mischievous tone to attract the consumer's attention."Reuse content