In search of inner calm and the ultimate natural high, growing numbers of Britons are braving freezing temperatures to swim in the great outdoors. From freshwater lakes and icy mountain pools to the thrashing waves of the sea, wild winter swimming – once the preserve of thrill-seekers and SAS training camps – is fast going mainstream.

Hundreds of clubs have formed across the country, according to the Outdoor Swimming Society, which has seen membership soar to more than 70,000 over the past two years, compared with only a couple of hundred a decade ago.

There are 200 mass swims this year – from Devon to the Scottish Highlands – but that is still unlikely to satisfy demand, according to Ella Foote, the group’s spokesperson. “They sell out like Glastonbury – we could fill these events twice over,” she said. And demand remains high despite hefty entry fees ranging from £50 to £150 a splash.

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The trend for winter dipping has taken off in the past year, Foote believes, because swimmers do not want to go to indoor pools, where there are “so many rules” and you “don’t get the magic of the outdoors”. “Plus, cold water has so many health benefits – it improves your mood, increases blood circulation, it helps with muscular pain,” she said.

“It feels bloody cold getting in but you become addicted to the effects of it – you feel great and buzzy all day,” said Mark Harper, who braves a chilly dip in the sea every morning with the Brighton swimming club.

His club has become so popular, he said, that they have closed it to new members and a second one has opened down the road. Further along the south coast, near Poole, Dorset, 59-year-old Karen Biles started winter swimming five years ago and has never looked back.

Facebook Twitter Pinterest Ella Foote takes to the water. Photograph: @ellachloeswims

“Ninety-nine per cent of my friends think I’ve completely lost the plot. But I don’t get migraines any more, I don’t get colds. I just feel much better all the time,” she said. And it’s not just the physical benefits: “You’re so busy concentrating on not getting too cold that anything that might have been troubling you is forgotten.”

Suzanna Cruickshank, who runs a business that takes people on swimming tours in western Cumbria, agrees. “You’re not in [the water] for as long in the winter, but the experience is magnified,” she said. This is the first year Cruickshank has taken clients on winter swims, primarily because it is something they have been asking for. Last week she took a group of women for an ice swim in a mountain lake.

A couple of days before the swim, she climbed up in the snow and cut a hole in the ice, to create a makeshift natural ice-bath that was big enough to swim in.

“First they were all saying, ‘Why am I doing this?’, screaming and inching in bit by bit, catching their breath,” she said. “Then all of a sudden it just dissipates and you see this delight on their faces.”