A mother of three died of sudden cardiac arrest after she plunged into a UK river for a $243 cold water therapy session, a UK court was told.

Kellie Jean Poole, 39, was pronounced dead at River Goyt in Derbyshire, England last April 25, in a session organized by UK-based Breatheolution, according to The Times of London.

The company’s website says the therapy can improve mental health, relieve stress and boost the immune system.

“The cold water therapy is proving to have a significant effect on clients, and although it can be tough including some uncomfortable sensations of the body, the benefits are incredible,” it says.

“Cold therapy and exposure to cold is now proving to be highly beneficial in so many ways, from sports injuries to even helping severe PTSD or anxiety sufferers step away from prescribed medication, not to mention what the cold delivers to our immune and nervous systems.”

It had been a mild day when Poole went for the session, but the participants were shivering even before they entered the river since they were only wearing bathing suits, Poole’s friend Victoria Fielding recounted at an inquest into her death on Tuesday, the UK Times said.

She said the group performed breathing exercises for about 15 minutes before they entered the water.

Breatheolution founder Kevin O’Neill then “went up to each of us individually and asked if we had any medical conditions,” Fielding said — though she noted they did not sign any waivers.

At first upon entering the river, Fielding said Poole was “enjoying” the therapy, “laughing and giggling.”

But she later started to complain about a headache, and O’Neill advised her to splash some cold water on her face as he scooped some water over the back of her head, her friend recounted.

Poole then fell forward into the water, Fielding said, and O’Neill performed CPR, but when paramedics arrived on the scene they pronounced her dead.

A post-mortem examination conducted by Dr. Abed Zaitoun recorded her death as sudden cardiac arrest caused by left ventricular hypertrophy — a thickening of the walls in the lower chamber of the heart.

Dr. Damien Kelly, a consultant cardiologist, explained at the inquest Tuesday that the cold water — which was recorded at 51.26 degrees Fahrenheit — could have caused an irregular heartbeat that “incapacitated” Poole.

When asked by senior coroner Peter Nieto whether he thought it was likely the cold water caused the sudden cardiac arrest, Kelly said, “Yes, I think that is what happened. It is difficult not to see it as relevant.”

Zaitoun also noted that Poole was considered obese based on her Body Mass Index, and her heart was heavier than expected.

“In my opinion, it is all related to the weight,” he said. 

“The larger the weight of the body, the harder the heart has to work for that body. That, in time, increases the weight of the heart.”

Nieto said he does not blame the Breatheolution founder for Poole’s death.

“I don’t see how it would have been possible for Mr. O’Neill to foresee that Kellie would get into the difficulty she did and sadly die on the riverbank,” he said at the inquest Tuesday, according to the Bristol Post. 

“Quite simply, he did not know that she had a very serious cardiac condition, and she did not know herself, so I don’t think it is possible to say her death could have been avoided.”

Still, Poole’s mother is now calling for more regulation of the activity — which has been lauded by celebrities like Kim Kardashian, Chris Hemsworth, Kendall Jenner and Madonna.

“I can’t believe that these activities are not regulated,” Diane Service said.

“It is a change that needs to come very soon,” she said, adding: “It is too late for Kellie, but not for someone else.”

Service said she knows her daughter’s death is “nobody’s fault, but I just can’t believe it.”

O’Neill also agreed that there should be more regulation, telling the inquest Tuesday: “I have witnessed every reaction possible in the cold in the last three years, and it does need regulating.”