YORKSHIRE Water has urged people not to swim in its reservoirs.
The plea came as children broke up for the summer holidays.
Water bosses said reservoirs were very deep, very cold and can had strong undercurrents.
Yorkshire Water safety manager Nikki Kemmery said: "It is important that adults, as well as children, do not ignore the signs warning them to stay out of reservoirs.
"It doesn't matter whether people are strong swimmers or not.
"Deep water is very cold and this can cause people to get into difficulties - often with tragic circumstances."
Yorkshire Water advises all people visiting reservoirs over the summer to pay attention to all signs.
"They are there to protect the public. Please don't ignore them," said Mrs Kemmery.