The sign at Highgate Men's Pond has some ominous advice for would-be winter swimmers.

"The advice of the Royal Lifesaving Society is that swimming in extremely cold or icy water is dangerous and should not be undertaken," it says.

Rescuers are at the ready at the Highgate pond

The water in the pond on the borders of Hampstead Heath, north London, is indeed "extremely cold" - 7 deg C, and ice is glinting around the edges.

And below the surface are rumoured to be giant pike and the unwelcome red swamp crayfish.

And yet, shunning official advice, I dive in, as about 40 others do every day at this time of year, regardless of the teeth-chattering conditions.

Threats from lurking wildlife are the last thing on my mind as the freezing cold robs me of my breath.

I have never swum so fast before. Thirty seconds later I'm back on the jetty, scrabbling for my towel, the hangover from the previous night's office party now just a bad memory.

Instead a glorious, vibrant glow was spreading from my fingertips to my toes.

Now where's the warm shower?

It's a feeling that hundreds of others around the country will be sharing on December 25. Throwing oneself into an icy pond, or the sea, for a few brisk strokes, is one of our more bizarre Christmas Day rituals.

The Highgate Lifebuoys swimming club will be holding its Christmas race at "11am sharp" this year. It has only been cancelled twice in the past, when the ice was too thick to break.

Members of the Brighton Sea Swimming Club swim every day including Christmas Day, when a crowd of around 1,000 turns out to witness this mass frolicking in the waves.

Britain's oldest continuously running sea-swimming club, it was founded in 1860. But today there is a nucleus of only 20 saltwater aficionados left.

Local newspaper reporter Adam Trimingham, one of the remaining hard core, calls it a "Brighton" thing to do.

"We do it for a laugh. It's all good fun and we have a brandy afterwards to warm us up again," he says.

Serpentine swimmers prepare for their Christmas Day dip

"I like the fact there's no reason for doing it. It's cold and completely bananas, but a very Brighton kind of activity."

Fellow club member, David Sawyer, suffers from arthritis and has to use fins, but can still swim from the Marina to the Palace Pier and back, a distance of three or four miles.

He sometimes fishes as he swims, carrying a bamboo cane with a line and tackle and tucking his catch - usually mackerel - into a bag around his neck.

He has traced the roots of sea swimming on the south coast back to the 1760s when it was considered a noble thing to do.

He says: "By braving the waves and exerting mind over matter, you were demonstrating your qualities as a person."

Brighton is bracing for Christmas swimmers

Noble or not, Christmas Day dipping is enjoyed in resorts around the country and even on the Continent thanks to the hardy Dieppe Penguins.

Among the toughest must be the men of Yorkshire who like to smother themselves with lard and dive into the North Sea off Scarborough.

But probably the most famous are the members of the Serpentine Swimming Club in London's Hyde Park, who indulge their devotion for a dip every Christmas in the l00yd race for the Peter Pan Cup, a competition which has been running for more than 125 years.

You can't take part unless you are a member of the club, but you can cheer competitors on. It all depends on your idea of fun.