Dyson gave the false impression that its Pure Hot + Cool fan was cordless in a TV ad, the Advertising Standards Authority has ruled.
One person complained to the watchdog that for the majority of the spot, no cord is visible.
In a firm response to the ASA, Dyson said the end shot of the ad clearly showed a cable coming out of the fan, which purifies the air in a room while heating or cooling it.
Dyson added that it was not aware of any major manufacturer marketing a cordless purifier in the UK, meaning it was reasonable to assume this would be one of the key features advertised – and so there was no "reasonable prospect" of the average consumer believing the device to be cordless.
But the ASA rejected these arguments, noting that the cable was thin and difficult to see in the final shot. It found that the ad breached the BCAP Code rules on misleading advertising and qualification.
Dyson increased the heat on the UK government earlier this year when the company announced plans to move its headquarters to Singapore, leading to criticism of founder Sir James Dyson, an enthusiastic proponent of Brexit regarded as a blowhard by political opponents.
Last week, it was reported that he had bought a £43m penthouse in Singapore, believed to be the most expensive flat in the country.