Tragic new details have emerged about the death of a three-year-old boy who passed away in his family's car during a heatwave that has scorched France.  

France is once again facing searing conditions today having endured its hottest ever 24-hour period on Wednesday. Yesterday, temperatures climbed past 44C in Pissos in the country's south-west. 

The boy died on Wednesday evening when his parents and firefighters failed to resuscitate him in the town of Saint-Gratien, public prosecutor Guirec Le Bras said.

The mother was having a nap with the couple's second child, an 18-month-old, while the father was working in a shed in the garden, he said, citing initial findings.

The father told his son to have a nap, but the child escaped his parents' supervision for at least 45 minutes and climbed into the car, where the child lock was activated.

'He then apparently shut himself in and became trapped in the vehicle before being found unconscious by his parents,' Le Bras said.

The boy's mother was heard screaming in horror for several minutes, neighbours told Le Parisien. 

One said: 'The mother screamed. It lasted about ten minutes. It was horrible.'

Another said: 'At first, I thought it was a woman being attacked.'

It was these awful screams that drew the attention of a nearby bus driver, who left his vehicle and tried to revive the boy before emergency services rushed to the scene. 

A neighbour recalled: 'We saw the mother screaming. Then she asked everyone to leave. She didn’t want people to see her like that.'

She was later taken to hospital in a state of shock. 

Julien Bachard, the mayor of Saint-Gratien, said: 'We can only be saddened by the death of a child, especially one so young. A three-year-old child, it’s terrible. People are deeply moved. Our thoughts are with the family.' 

The horrific deaths follow the passing of two young siblings who were left in a hot car earlier this week. 

The brothers, aged just four and two, were found unresponsive by their mother, 33, on Monday afternoon in the town of Carpentras, southern France, in a car parked outside their grandmother's house. 

They suffered cardiac arrest as temperatures reached a sweltering 40C, and while services were called to the scene, resuscitation efforts were unsuccessful. 

And an elderly British woman died at a French campsite amid the scorching heat. 

She fell ill while staying at the Baie D-aunis campsite with her husband, in the coastal town of Tranche-sur-Mer. 

After she took ill, emergency services, including four ambulances and two police cars, were rushed to the scene.

At least 48 people have drowned across the country since the heatwave began with extreme heat closing nuclear reactors and triggering forest fires. 

Meanwhile, the freak Saharan heat dome is causing chaos across the rest of Europe including in Germany, the Netherlands, Belgium, Italy and in Spain where 212 have died in three days.  

The heatwave is being driven by ​a weather pattern known as an Omega block, pushing temperatures as much as 18C above normal.

The phenomenon resembles the shape of the Greek letter Omega, with a bulbous middle trapping in heat over regions for extended periods, with cooler weather on its fringes.

Weather agencies have warned that extreme heat could endanger lives as much of western Europe bakes under a 'heat dome'. 

France on Wednesday recorded the hottest day since measurements began in 1947, the national average temperature reaching 30C.

Paris mayor Emmanuel Gregoire earlier on Thursday reported that deaths were on the rise in the capital.

The Ministry of Health later confirmed that there had been 25 heat-related cardiac arrests in the last 24 hours in Paris. 

The extreme heat has also led three nuclear reactors to close in France as high temperatures across the country reduced access to water needed to cool the plants. 

The Golfech reactor has been closed since Monday, while the Nogent-sur-Seine nuclear power plant in Aube shut this morning due to 'external causes related to the environment'.

A third was closed in the late morning in Bugey.

Forest fires also broke out in Spain and France. 

Nearly 200 firefighters were deployed to put out flames spreading through the Landes de Gascogne pine forest in southwestern France. 

In Spain's Catalonia region, an inferno tore through 16 hectares of forest in Sant Quirze de Safaja.  

At least 101 million people are expected to experience hot temperatures on Thursday, including 50 million in France. 

The country has been the most impacted, where around 63 million people have experienced temperatures of more than 30C. 

The heatwave has also been linked with 212 deaths in Spain between Sunday and Wednesday, estimates from a public institute show.

The MoMO monitoring system compiles daily death statistics in Spain and calculates the difference in mortality by comparing them with the levels forecast based on historical records. 

Meanwhile, the UK recorded its hottest June day on Wednesday, with 36.1 reported at Gosport in southern England. 

The national weather forecaster issued a red alert for heat in much of central and southern England, as well as Wales.

The bodies of three young men were recovered from the Marchfeld Canal near Gerasdorf in Austria on Thursday, after they were initially reported missing.

The men, between 25 and their early 30s, are believed to have had an accident during a stand-up paddleboarding trip and drowned, according to Kronen Zeitung.

Authorities warned people to take extra care when swimming in unsupervised areas, such as rivers or lakes, following the deaths of at least 48 people in France over the past week.

French footballer Kenzo Kies has died from drowning in the Rhone River after he had gone for a swim to cool down from the blistering heat.  

Kies, 21, who played for Guingamp in Ligue 2, was in a critical condition when he was pulled from the water on Monday, and later died in hospital.

Kies and three friends entered the river near Lyon. Three people were pulled to safety by emergency services, but Kies was the last to be found.

More than 1,000 schools in England have closed due to the heat, and many train services were cancelled, with passengers being urged to avoid nonessential travel in areas covered by the warning. 

In France, Italy and Spain, more than 100 million people were warned to be extra vigilant about the dangers of the heat wave.

With the mercury rising, many of France's major attractions, including the Eiffel Tower and the Louvre Museum in Paris, have restricted visiting hours. 

Schools and transportation schedules were also upended.

In Italy, 16 cities, including Rome, Milan, Florence and Turin, were under heat alerts.