Forecasters have put 15 French departments on the highest state of alert for extreme temperatures while in Britain the government was accused of failing to take seriously the impending heat emergency as forecasters warned that lives were at risk.
The heatwave, spreading north, began as the second to engulf parts of southwest Europe in weeks, and blazes burning in France, Greece, Portugal and Spain have destroyed thousands of hectares of land and forced thousands of residents and holidaymakers to flee.
Scientists blame climate change and predict more frequent and intense episodes of extreme weather such as heatwaves and drought.
In France's Landes forest, in the southwest Aquitaine region, temperatures "will be above 42 degrees Celsius" (107 Fahrenheit) on Monday forecaster Olivier Proust said.
And Brittany, which until recently has escaped the worst of the heat, could register temperatures as high as 40C, say experts, which would be a record for the region.
In the southwestern Gironde region, firefighters over the weekend continued to battle to control forest blazes that have devoured nearly 11,000 hectares (27,000 acres) since Tuesday.
'A heat apocalypse'
Spanish authorities have reported around 20 wildfires still raging out of control in different parts of the country from the south to Galicia in the far northwest, where blazes have destroyed around 4,500 hectares of land.
Spainsh authorities announced that a member of the fire service died on Sunday while working to extinguish forest fires at Losacio in the northwestern Zamora region. The fires have already killed several civilians and emergency personnel since last week.
The wildfires in France forced more than 16,000 people -- residents and tourists combined -- to decamp. Seven emergency shelters have been set up for evacuees.
France's interior ministry announced it would send an extra three firefighting planes, 200 firefighters and more trucks.
"In some southwestern areas, it will be a heat apocalypse," meteorologist Francois Gourand told AFP.
The chapel of a historic hospital in the southeastern city of Lyon, Grand Hotel Dieu, offered refuge to tourists on Sunday including Jean-Marc, 51, who was visiting from Alsace.
"We came back to admire the place, but we can't leave, it's too hot outside. We say a prayer before the fire!" he quipped.
French cyclist Mikael Cherel, taking part in the Tour de France's 15th stage between Rodez and Carcassonne in southern France on Sunday, described "very, very difficult conditions".
"I've never known such a hot day on a bike. It really was no picnic."
In Portugal, almost the entire country remained on high alert for wildfires despite a slight drop in temperatures, after hitting 47C -- a record for the month of July -- on Thursday.
'Risk to life' in UK
Only one major fire was burning on Sunday in the north.
The fires have killed two, injured around 60 and destroyed between 12,000 and 15,000 hectares of land in Portugal.
In Britain, the weather office issued a first-ever "red" warning for extreme heat, cautioning there was a "risk to life".
The Met Office said temperatures in southern England could exceed 40C on Monday or Tuesday for the first time, leading some schools to say they would stay closed next week.
Eyebrows were raised, however, by comments from Deputy Prime Minister Dominic Raab appearing to welcome the likelihood of temperatures topping 40C and Prime Minister Boris Johnson missing a meeting about the government's response to the heatwave.
"This isn't like a lovely hot day where we can put a bit of sunscreen on, go out and enjoy a swim and a meal outside," College of Paramedics chief executive Tracy Nicholls told Sky News.
"This is serious heat that could actually, ultimately, end in people's deaths because it is so ferocious," she said.
The UK capital is expected to see the highest temperatures and mayor Sadiq Khan advised Londoners only to use public transport if "absolutely necessary".
Ambulance services are on crisis footing, and some schools in southern England have already said they will stay shut.
In the Netherlands, the mercury is set to reach 38C in parts of the country on Tuesday.