His powers over people's minds are well known but the hypnotist Paul McKenna may be less capable of permanently changing the bodies of his clients, according to a report by the Advertising Standards Authority (ASA).

An ad for a Paul McKenna event is criticised today for implying participants were guaranteed permanent weight loss. A photo of the hypnotist featured in the ad along with the claim his programme was: "The most effective weight loss system available. Lose weight and keep it off."

The ASA found that the regional press advert was in breach of the advertising code. Paul McKenna Training – which advertised the weight loss event – could not back up the implication that participants would permanently lose weight, the watchdog said. It told the company not to repeat the claim and said it should seek the advice of experts before making similar claims in future advertisements.

The ASA accepted that a "high proportion" of participants had reported losing weight after attending a Paul McKenna event. But the watchdog said this was "inadequate to support the implication that all participants would lose weight and keep it off forever".

Paul McKenna Training sent the ASA surveys of previous weight-loss event attendees which showed 71 per cent had lost weight.

The hypnotist's company agreed not to repeat the advertising claim until it had independent research to back it up.