At least 13 people have died in open water over the heatwave period and water safety organisations are concerned that the toll may climb as schools break up for the summer holidays.

At least four people under the age of 17 have died in open water since Saturday, with the youngest confirmed as 13-year-old Robert Hattersley, who got into difficulties in the River Tyne near Ovingham, Northumberland on Sunday.

A 14-year-old boy is believed to have drowned in west London, and two 16-year-old boys have died in incidents since Saturday. One has been named as Sean Norbert Anyanwu, who died in Bray Lake near Maidenhead, Berkshire, on Monday.

Matt Croxall, senior head of membership and field operations at the Royal Life Saving Society (RLSS) UK, warned: “What we now need to be aware of as well is that some schools will have already broken up in Scotland and Ireland. In England, schools are breaking up or have broken up around this time too.

“So it’s a lot more likely that young people will be out and whilst there’s this hot weather then they’ll be seeking ways to cool down as well and will often take unnecessary risks.”

The organisation encourages people to swim at supervised sites, and if that is not possible, to enter the water gradually to avoid cold water shock.

“We’d encourage people to enter the water gradually, particularly whilst we’re in a heatwave right now. That difference between the air temperature and diving into very cold water can cause people a real problem and can cause that cold water shock response,” Mr Croxall said.

The organisation also warned of “hidden hazards” under the water at inland sites such as on riverbeds, which pose risks to swimmers.

Cumbria Fire and Rescue Service tweeted a picture of wooden spikes sticking out of a river bed, urging swimmers to “think twice” before jumping into open water to cool off.

The service said: “There are many hidden dangers beneath the surface of our waters.

“Due to the hot weather, river levels have dropped significantly, and this photo shows wooden spikes which the water usually covers!

“Please think twice about jumping into our waters from height.”

The National Fire Chiefs Council (NFCC), the representative body of UK fire and rescue services, issued a statement on Tuesday urging young people to “look out for their mates” and “not take risks” with open water.

In a statement the council said: “NFCC is aware of 13 reports this month of accidental drownings. This includes seven young men from across the country between the ages of 13 and 17, drowning after attempting to escape the heat by heading to rivers, quarries, canals and lakes.

“Many people do not have experience of swimming in open water and may not be aware of the risks. Despite the soaring temperatures, the water can remain cold. Jumping in for a swim can lead to cold water shock and accidental drowning, regardless of swimming ability.”

The RLSS expects the death toll to rise as further deaths are confirmed.

“To be honest, there’s very likely to be more to come, because often there’s a lag in drowning statistics where we hear a few days later of an incident, but we’re certainly well into double figures, we can say,” Mr Croxall said.

A spokesperson for the RNLI said: “As a charity, we exist to prevent drowning so that nobody suffers the devastating loss of a loved one in this way, and our thoughts are with their family and friends at this time.

“The RNLI’s advice to anyone planning a visit to the coast is to choose a lifeguarded beach where possible and swim between the red and yellow flags which is the safest area as it is constantly monitored by our highly trained lifeguards.

“A lifesaving skill we ask everyone of any age to remember is how to float to live if they find themselves in difficulty in the water. Lean back, using your arms and legs to stay afloat. This will allow you to control your breathing, then call for help or swim to safety. The technique has been proven to save lives.”