The owners of two closed water sites has issued a warning following reports of people swimming during hot weather.

Affinity Water says swimming at Hilfield Park Reservoir near Elstree and Stockers Lake in Rickmansworth is "extremely dangerous".

On Thursday (July 22), the Royal Life Saving Society UK says it was aware of 17 people who had died in the water across England and Northern Ireland between July 17 and 20, and urged swimmers to take care.

On Tuesday, a man drowned after getting into difficulty at Stanborough Lakes in Welwyn Garden City.

Although cooler weather is forecast over the coming days, Affinity Water has issued a warning to anyone thinking of taking a dip into its sites.

Kevin Barton, head of external communications at Affinity Water, said: "We realise how tempting, especially during hot spells, it must be to swim in lakes to try and cool off but the sites we own are extremely hazardous for this activity.

"With so many tragic deaths this year already in the UK in open water, we are kindly reminding people of the severe dangers of swimming at our locked and managed sites.

"Please do come and enjoy the nature reserve at Stockers Lake but please, for your own safety, do not enter the water. The Hilfield Park reservoir is a locked site and is not open to the public."

Affinity Water says its Hilfield Reservoir and Stockers Lake sites have very cold water and strong currents, with obstructions and debris in the water.

Concerns have also been raised by Herts and Middlesex Wildlife Trust, which manages the two water sites, about habitats and wildlife being disturbed or harmed.

Josh Kalms, people and wildlife officer, said: "We want people to enjoy the nature reserves we care for in a safe and respectful way. Swimming and boating is prohibited on the nature reserve all year round in order to protect the sensitive habitats of the wildlife found there.

"Stocker’s Lake is one of very few undisturbed refuges for wildlife in the Colne Valley. Sadly we’ve had instances of visitors swimming in the lake and disturbing nesting birds which is a criminal offence."

Mr Barton added: "With open water swimming becoming more popular, there are a number of supervised open water swimming clubs and qualified instructors at designated safe sites across the country and we would advise anyone interested in this to contact these organisations to find out more.

"However, entering a prohibited or locked reservoir can put your life in extreme danger."