Warnings about the “lurking dangers” in Alton Water are being issued after reports of groups of people jumping in the reservoir to cool off in the hot weather.
Anglian Water, who own the reservoir, are receiving reports of people ignoring their messages that it is dangerous and illegal to jump into reservoirs unsupervised.
The water company is issuing a plea to remind people not to be tempted to swim during the hot weather this summer.
Anglian Water's David Hartley said: "When the sun comes out we know the public, particularly young people, are more inclined to gather around waterways to socialise to enjoy the weather.
"Although our reservoirs might be a tempting place to take a dip to cool off in the warm weather, there are many hidden dangers in them lurking below the surface.
"Swimming in our reservoirs is illegal, unless it is in a properly designated and supervised area.
"Reservoirs are there as storage so we can supply customers with drinking water.
"This means they are very deep and the water can be extremely cold in places, even when it's hot outside.
"This sudden change in temperature can cause cold shock which takes your breath away and makes it hard to swim.
"There are strong currents in the water and hidden machinery lies just below the surface, not to mention the mud and weeds which can trap even the strongest of swimmers."
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