JUMPING into very cold water can cause the body to go into shock, a doctor has explained.

Dr Tom Smith, the Lancashire Telegraph’s health expert, said the sudden change in temperature can cause muscles to spasm, meaning swimmers can drown or suffer heart failure very quickly.

He said: “People think that going swimming in a reservoir might be the same as swimming in a swimming pool, but it is not.

“If you have a deep reservoir, the water stays much colder. So instead of going into a swimming pool of 22C to 23C, you are jumping into water that is only 10C to 15C.

“It is quite a shock to the system.”

The air temperature on Sunday was in the low 20Cs.

The doctor explained that when the body is hot, the blood vessels come to the surface of the skin to allow it to sweat and cool down.

But when you are suddenly plunged into cold water, the body struggles to keep the blood flowing.

Dr Smith said: “When your body temperature is high, it is a much, much bigger shock than people think.

“What can suddenly happen is your heat regulation system shuts down, then your muscles go into cramp and cannot move.

“A sudden change in your body’s way of opening and closing smaller arteries creates a sudden change in blood flow to vital organs. Your heart can suddenly stop.”

Dr Smith said this was the most common cause of death for people killed in open water, rather than drowning or hypothermia.