A spring water hailed as a wonder cure-all by the celebrity chef Clarissa Dickson Wright is under investigation by health authorities over its promoters' claims that it could cure eczema and psoriasis.

Lakeland Willow Spring Water, which is drawn from an ancient underground well in the Lake District, has become a phenomenon among its fans. Adherents claim it cures cancer and arthritis. Even racehorses are alleged to win races after drinking it.

But the water - which shot to fame last year after Ms Dickson Wright claimed on breakfast television that it treated a benign cyst on her breast and a "gungey toe" - is now in trouble with the consumer safety authorities.

The Food Standards Agency and Cumbria County Council have launched inquiries after Lakeland Willow began a new promotion last month which alleged that people who drank the water saw "remarkable improvements to a whole host of skin complaints" - including the often serious illnesses eczema and psoriasis.

The FSA, which enforces food safety and labelling laws, and trading standards officials in Cumbria will investigate whether this breaches strict rules that bar firms from claiming that food and drinks can cure, treat or prevent a medical condition.

The case has highlighted concerns at the FSA over a surge in the sales of so-called "aquaceuticals" - the growing number of mineral and spring waters which are controversially claimed to have health-giving qualities. Some claim to have spiritual strengths, such as Kabbalah Water, a product blessed by a rabbi and endorsed by the singer Madonna.

Drinks such as Danone Activ, launched three years ago, boasts that its added calcium can help to prevent osteoporosis, and Nestlé's Contrex alleges it helps weight-loss - claims that health experts and medical charities treat with deep scepticism.

Lakeland Willow claims its properties are derived from traces of salicin, a mineral that was once the active ingredient of aspirin and is found in the bark of white willow trees. The Cartmel Valley, the source of its spring water, was once covered with white willow forests - trees that now form much of the peat that carpets the valley and through which the spring water is filtered.

David Jones, the company's managing director, admitted he had no scientific evidence to prove it could treat skin diseases, but said he had been struck by a flood of "superb" unsolicited testimonials from customers since the bottled water was launched a year ago.

The company now sells nearly four million litres a year - chiefly due to word of mouth about its alleged properties, he said. "It's spooky, what's happening. It's a bottle of water at the end of the day, but there's no question it seems to be working."

The inquiry over Lakeland Willow will focus on whether the firm and its marketing agency, Kudo Communications, have kept within the law by basing its marketing on testimonials from its customers rather than making outright claims of its own.

Mr Jones said the firm is trying to set up trials on the water's properties with the Glasgow Homeopathic Hospital. However, Phil Thomas, a food safety expert for the Trading Standards Institute, said Lakeland had "stretched" the law "as far you can possibly stretch it without making a medicinal claim".

Skin disease charities were also very uncomfortable with the claims, partly because sufferers who bought the water might stop using conventional treatments but then find that the water had no effect at all.

The British Dermatological Society said it had never seen any proof that this water could treat skin conditions. Margaret Cox, chief executive of the National Eczema Society, said: "My advice is to exercise extreme caution over something marketed exclusively on the back of individual testimonies, because, in truth, it has no real grounds behind it."