Two organisations are warning people about the dangers of swimming and playing in rivers and lakes ahead of a Met Office warning about record temperatures set for this week.

The mercury is set to hit a high of 34 degrees in the same week that primary and secondary schools break up for the summer holidays, which some fear will encourage youngsters to cool off in open water.

Anglian Water has issued a warning about the hidden dangers lurking below the water's surface.

The water company, in partnership with its new ambassador Ben Saxton, has launched a reservoir safety campaign to warn people of the dangers of swimming in reservoirs.

Ben, who competed for Team GB at the 2016 Olympic Games in Rio de Janeiro and is part of the British sailing team, started his sailing career as a youngster at Anglian Water's Grafham Water reservoir. As ambassador for the company, he is now encouraging people to stay safe at reservoirs such as Alton, Rutland, Grafham and Pitsford and any other body of open water.

He said: "I have been sailing and playing in the water since I was four years old. The open water is exciting and fun but I'm always very aware of the danger it can pose too. The depth and cold temperature mean even on sunny days you can get into trouble. Be sensible, be safe and keep an eye on your mates."

He added: "When you're enjoying yourself with friends, it's hard to remember that playing in the water can be dangerous. Respect the water. You never think it could happen to you, but circumstances can change quickly if you're not careful."

The advice follows a series of instances over the last few years where two young people tragically lost their lives after swimming unsupervised in rivers and reservoirs in East Anglia. Anglian Water's David Hartley said: "Swimming in our reservoirs is illegal, unless it is in a properly designated and supervised area.

"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. The unexpected temperatures can cause cold water shock and make it difficult for anyone."

Cambridgeshire Fire and Rescue Service is also issuing a reminder about water safety. The service launched its #RememberRony campaign

in July, 2015, a year after the death of Huntingdon teenager Rony John, who drowned while swimming in the River Great Ouse at Huntingdon.

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Rony, aged 15, a former St Peter's School pupil, and some of his friends were playing in the river at Hartford on the first day of the summer holiday on July 24, 2014 when Rony, who not a strong swimmer, got into difficulties.

Despite efforts from his friends, firefighters and two men who jumped in to save him, Rony drowned.

A fire service spokesman said: "As the weather heats up and we see record temperatures in some areas of the county, it can be tempting to take a dip in open water to cool off. Areas of open water contain many hidden dangers beneath the surface and can be deeper than anticipated, plus, despite the high temperatures, the water can be exceptionally cold. The body could easily go into shock causing a person to panic to try and get to the surface.

"Anyone finding themselves in this position should try to remain calm and allow their body to float to the surface of the water, then swim to the closest bank to safety.

"If you find someone in need of help who is struggling in open water, or if a beloved pet has found their way into trouble, we urge you to not enter the water yourself. Call 999 and ask for the fire and rescue service to help. Provide as much information as possible, particularly your location. To help with this, have the what3words location app installed on your phone, which will help you pinpoint your whereabouts using a unique three word combination that can be given to the emergency call handler.

"We want people to enjoy the hot weather while it's here, but to do so as safely as possible. Please stay away from open water and don't be tempted to go for a dip, visit a local lido or swimming pool where there are lifeguards on duty."