People have been warned of the dangers of swimming in open water after multiple deaths during the recent heatwave.

The Royal Life Saving Society (RLSS) issued a warning following the fatal incidents, most of which involved children, saying "warmer weather unfortunately sees an increase in accidental drownings".

At least seven young people died in the water, the youngest being 12-year-old Junior Slater, who died in a Lancashire river on Tuesday afternoon. A man in his 60s and a woman in her 70s also died in the water.

RLSS warned water temperatures remain very cold despite increased air temperatures, and cold water shock can make swimming difficult and harder to get out of the water.

A leading network for drowning prevention has recommended swimming in supervised locations with lifeguards where possible and entering the water slowly to reduce the risk of cold-water shock.

Cold-water shock is an involuntary pshysiological response when the body is suddenly immersed in water.

It causes blood vessels in the skin to close, which increases blood flow resistance, and as a result forces the heart to work harder and causes blood pressure to increase.

According to the Royal National Lifeboat Institution (RNLI), anything below 15C is defined as cold water, and average UK and Ireland sea temperatures are 12C, and some rivers can be colder even in summer.

Speaking to BBC Breakfast on Wednesday morning, Jim Bridge from Water Safety Partnership urged those who find themselves in difficulty in the water to "float to live".

"So lie on your back with you head back, with your ears submerged, use your arms and legs to stabilise yourself and just let your breathing come back to a normal breathing pattern.

"And when you're feeling confident that you've gotten used to the water and your breathing's normal, you can shout for help or swim to save yourself."

On Tuesday, the UK recorded the hottest day in May with temperatures reaching 35.1C at Kew Gardens in London.

For two days in a row, parts of south-east England went above 34C.

Nearly all weather stations across England and Wales also broke their local May temperature records on Monday and Tuesday.

The Met Office says a heatwave happens when an area reaches a certain high temperature for at least three days in a row.

Wednesday brings a reprieve from the record-breaking temperatures which have likely peaked, as cooler air moves over northern and eastern areas of England.

However, National Rail warned until Thursday services could be altered as the hot weather "can cause overhead lines to expand and sag, can cause line-side fires and it can resort in rails to buckle", a spokesman said.

Speed restrictions could also be imposed. Currently there is major disruption between Lancaster and Oxenholme Lake District which is expected until the end of the day.

Train companies affected include Avanti West Coast, Northern and TransPennine Express.

The Met Office has also issued a yellow thunderstorm warning for much of Wales, south-west England and parts of the Midlands from 15:00 to 23:59 on Wednesday.

It could bring torrential downpours of rain, frequent lightning, large hail and strong, gusty winds.

But over the next few days it will still remain hot in many areas and health officials confirmed heat health alerts would be extended for 24 hours.

The UK Health Security Agency (UKHSA) said amber alerts for the South West, South East, London, East and West Midlands, and East of England will remain until 17:00 on Thursday, with yellow heat-health alerts for the North West and North East.

Chef Tony Church says staff took the decision to close on Sunday evening to keep everyone safe.

How to make sure your pets are cool and comfortable during periods of hot weather.

Welsh Water will pay Β£20 a day to customers in Rhondda Cynon Taf affected by the five-day boil order.

People living in parts of Northamptonshire and Bedfordshire report their taps running dry.

Wales has recorded its hottest May day in over 80 years, with a temperature of 32.9C.