They think it is just harmless fun, playing on the ice-covered reservoir.

But today these youngsters - and many others beside - were warned they were playing a dangerous game and putting their lives on the line.

These children were caught standing on ice, throwing bricks and stones across lakes at Chellow Dean Beck in Bradford.

And a leading doctor today warned that the potential consequences of their fun included fatal heart attacks as well as hypothermia and drowning if the ice gave way.

The stark warning came as temperatures across the district plunged below zero.

Bradford Accident and Emergency consultant Tony Shenton said he first became concerned after a colleague spotted people walking on the canal in Shipley.

"However tempting it may look, members of the public are putting themselves at considerable risk if they succumb to that temptation," he warned.

"If you walk on ice, particularly if it's melting, and you plunge into water at three or four degrees centigrade, there's a significant chance you won't come out.

"Water at that temperature can trigger a cardiac arrest from which it's difficult to resuscitate a victim."

One man has already died after falling through ice on a reservoir in Edinburgh.

The Telegraph & Argus asked Yorkshire Water if they were aware of the problem after spotting these children on the reservoir.

A spokeswoman for Yorkshire Water said staff had been alerted to youngsters in the area but added: "We urge people to keep away from reservoirs at all times. They are dangerous when they're not frozen but extremely dangerous when they are. People are risking their lives by ignoring the warning signs."

Roger Vincent, of The Royal Society for the Prevention of Accidents, also made a plea to Bradford folk to stop the winter madness.

"A lot of people are likely to go out walking at the weekend and with the cold spell continuing, we are extremely concerned that there could be more tragedies in the next few days," he said.

"Even testing the thickness of the ice with your toe puts you in danger. It is easy to slip from the bank and go through the ice and if the ice appears to take your weight, the temptation is always to take another step forward where the ice could be much thinner."

British Waterways manager for the Leeds and Liverpool canal, Alan Bates, said frozen canals were sometimes covered with a dangerously thin, uneven layer of ice.

He said. "Even when the ice thickness is greater and temperatures rise, it is hard to tell when and where the thaw is occurring. This makes iced over canals potentially very dangerous, particularly to children."

A spokeswoman for West Yorkshire Metropolitan Ambulance Service said there had not been many reports of accidents on ice, but that there had been an increase in road crashes due to dirty car windscreens.