It is a fashionable middle-class lifestyle trend, with a growing band of proponents promoting its mental and physical health benefits.
But cold water swimming could be putting its thousands of new converts in danger as they swim for longer than their bodies can take.
This week, the Parliament Hill Lido in Hampstead, north London, issued a warning to inexperienced cold water swimmers, after reporting at least one person a day has been struck down by hypothermia since the wintry weather started in earnest.
The lido, whose frequent visitors include Alastair Campbell and former minister Jo Johnson, is open 365 days a year for unheated outdoor swimming.
For the first time, it issued a warning to swimmers in a post on its Facebook group page this week, saying: “The lifeguard team have asked you all to be careful and to shorten your swim from now on. The water temperature has dropped 5 degrees in a matter of days and some people are still insisting on staying in the water as long as they were last week.
“We have had at least one hypothermic incident every day for the last week. Please, please be careful and don’t stay in the water too long.”
‘As you start feeling a tingle, get out’
One of the pool workers told The Telegraph: “Over recent years it has become more and more fashionable to swim in cold water and that has meant we have had more and more people doing it.
“But not everyone is experienced and undertaking proper acclimatisation. And not everyone knows the signs – as soon as you start feeling a tingle you really should get out.
“We generally say for guidance that the number of the temperature equates to the amount of time it’s safe to spend in the water – so one minute at 1 degree, two at 2, and so on. This week the temperature has been down to 8 degrees but we’ve had people continuing to try to use it to exercise.
“We are all now on alert for people showing signs of hypothermia and we have a protocol to get them dry and warm immediately for their safety.”
A spokesman for the Hampstead Heath charity, which runs the lido for the City of London Corporation, said: “We are urging swimmers to follow our cold water swimming guidelines, especially during the recent drop in temperatures. There are risks associated with cold water swimming, even for regular winter swimmers. We are advising people to swim with care and only spend a short amount of time in the water.”
They added: “The Parliament Hill Lido is staffed with fully trained lifeguards to help keep swimmers safe.”