Before I visited Biarritz, all I knew about thalassotherapy was that it involved a terrifying-sounding "fireman's hose" treatment. The victim stands naked at one end of a narrow tiled room while a jet of high-pressure water thunders over their flesh. It sounds like something out of The Story of O, especially when the temperature is turned right down to "stimulate the skin". However, my four-day break proved to be anything but a punishment. It was a hedonistic time of wallowing in hot seawater baths, moving in a daze from massage to massage, and being wrapped in seaweed or plastered with bubbling mud. Even the dreaded "fireman's hose" was strangely invigorating, leaving my skin tingling and, I hoped, my flab a little more toned.

Before I visited Biarritz, all I knew about thalassotherapy was that it involved a terrifying-sounding "fireman's hose" treatment. The victim stands naked at one end of a narrow tiled room while a jet of high-pressure water thunders over their flesh. It sounds like something out of The Story of O, especially when the temperature is turned right down to "stimulate the skin". However, my four-day break proved to be anything but a punishment. It was a hedonistic time of wallowing in hot seawater baths, moving in a daze from massage to massage, and being wrapped in seaweed or plastered with bubbling mud. Even the dreaded "fireman's hose" was strangely invigorating, leaving my skin tingling and, I hoped, my flab a little more toned.

Thalassotherapy is, in fact, a series of relaxing treatments designed to revitalise, invigorate and even heal the body using seawater, sea mud and seaweed. The theory is that all the good minerals and micro-organisms present in seawater cross into the skin, while all the toxins pass out. "If we put you in water for 20 minutes, the pores open and allow the transfer of minerals by osmosis," says Jacques Brion, director of Miramar hotel in Biarritz, which has a thalassotherapy centre in its basement. "If we heat the water to 34C, it speeds up the process."

Seawater's curative and feel-good benefits have been known about for centuries; the Romans and the Greeks built baths on the British coast and the Victorians rushed to seaside resorts in droves. But it wasn't until the second half of the 20th century that the techniques of seawater therapy became a series of formal treatments, and though the British Isles are surrounded by the sea, it wasn't at home that this happened, but in France.

Now an annual treatment is almost de rigueur for the French, and it stands a good chance of catching on among the ever spa-conscious British, with Ryanair offering cheap flights to Biarritz. Thalassotherapy might have been born in Brittany, but in Biarritz it comes with a hefty dose of southern sunshine and a choice of four centres, each specialising in different treatments.

Compared with English spas, these have a strong clinical feel, with lots of tiles, open spaces and a muddy seaweedy smell that, together with the fluffy robe and slippers everyone wears, makes you feel you're more of a patient than someone going for a pampering session.

The stark cubicle with a blue bath at its centre into which I was whisked within minutes of my arrival at the Atlanthal thalassotherapy centre didn't look very welcoming. The red emergency button on the side of the bath was even a little alarming. But after lying back for 15 minutes as the jets played on my body, I soon realised that as well as being good for me, this was sheer bliss. Helping my lymphatic drainage along and generally toning my body proved to be a sensory delight that left me staggering out to the horizontal shower, where hot seawater was sprayed over my back just long enough to leave me dreaming I was in a tropical shower.

Though seawater is good for you, seaweed, it turns out, is better. Harvested in Brittany and left to dry on the sand dunes, it concentrates those good oligo-elements in the seawater 50,000 times. Rich in iodine, iron, manganese and calcium, it remineralises the body and helps with toning and rheumatic pains, gets rid of toxins through sweating and leaves your skin silky smooth. So I gave myself over to being plastered in green slime before being wrapped in something akin to clingfilm and left to bake. Then it was on to the mud, which an assistant spread on a sheet of what looked like foil before plastering it on to my back. The body's heat causes the mud to blister, which gives you the delicious feeling of wallowing in warm, bubbling mud.

By this time, the clinical surroundings of the centres were blurring into a delirious haze and the question of whether to keep swimwear on or off during the treatments didn't seem such an important one as I went for one of the best massages I've ever had. In France, masseurs do a three-year physiotherapy course and their experience and knowledge come across instantly in their touch.

But while good, the dry massages paled into insignificance compared with the underwater ones at the Miramar's luxury thalassotherapy centre, where water under high pressure is directed on all parts of the body. Starting at the feet for circulation, the jet moves to the hands for the nervous system, the legs for cellulite, the stomach for digestion and the back for stress.

To reap the benefits, most guests stay for a week, having four treatments a day, but a quick weekend pick-me-up still does wonders. As well as special treatments for men, the centres offer individual packages for everything from stopping smoking to helping young mothers to regain their shape. One of the most popular options is the weight-loss programme, which can be accompanied by exotic, satisfying menus based on 1,200 calories a day (though it seems a shame not to sample the glorious seafood while you're here).

The weight-conscious can also take advantage of the spa gyms. But though the Miramar's is tempting, with its running machine looking out to sea, I preferred to lounge around on the day beds looking out at the wide stretch of sandy beach. Why on earth would anyone bother with a gym when you can let the water do all the work for you?

The facts

Getting there

Ryanair (08701 569 569; www.ryanair.com) flies daily to Biarritz from £45 return.

Being there

(Prices are per person for six-day packages in low season on a half-board basis with four treatments a day).

Hotel Atlanthal, Anglet (00 33 559 527575; www.atlanthal.com) A simple three-star hotel with sea views, adjacent to a smaller two-star hotel. The mud treatment is a speciality. Prices: from £494 two star and £567 three star.

Thalassotherapy Serge Blanco, Hendaye (00 33 559 513535; www.thalassoblanco.com) This centre has three-star and four-star rooms and specialises in treating sports injuries. Prices: from £696 three star and £678 four star.

Hotel Miramar, Biarritz (00 33 559 413000; www.sofitel.com) A beautiful four-star hotel, this is the most luxurious of the four centres and has less of a clinical feel. Prices: from £666.

Hotel Hélianthal, St Jean de Luz (00 33 559 515151; www.helianthal.fr) This three-star hotel is in a quaint old Basque village. The spa looks like a school swimming pool, but it has a fabulous view of the beach. Prices: from £596.

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