The United Kingdom went on high alert to brace for its hottest day on record on Monday, with temperatures forecast to hit 40 degrees Celsius (104 degrees Fahrenheit) for the first time as deadly wildfires raging in parts of southwest Europe showed no sign of abating.

Scorching temperatures forced train companies to cancel services, schools to close early and ministers to urge the public to stay at home.

Much of Europe is baking in a heat wave that has pushed temperatures into the mid-40s Celsius in some regions, with wildfires raging across the tinder-dry countryside in Portugal, Spain and France.

Britain's government triggered a "national emergency" alert as temperatures were forecast to surpass the 38.7 degrees Celsius recorded in the Cambridge University Botanic Garden in 2019 on Monday and Tuesday.

"We've got a difficult 48 hours coming," Kit Malthouse, a minister in charge of government coordination, told BBC radio.

London's metro network imposed temporary speed restrictions for Monday and Tuesday, meaning it would run a reduced service with journeys taking longer than normal. It urged commuters to only travel if essential.

The national rail network also urged passengers to stay at home and said some services – including a key route between northeastern England and London – would not run during parts of Tuesday.

Jake Kelly from Network Rail said he hoped normal operations would resume on Wednesday, when temperatures are forecast to fall, but that would depend on "the damage that the weather does to the infrastructure over the next couple of days."

High alert

The government urged schools to stay open but many were due to close earlier than usual, normal uniform demands were ditched and end-of-term sports days were cancelled. Some schools were shut, resorting to lockdown-style online lessons.

And at least one major zoo, at Chester, said it would close for two days, while London Zoo and Whipsnade Zoo said many animals would be able to retreat to "cool zones" and some exhibits might be closed.

Some factories also brought their opening hours forward, to prevent workers on the hottest jobs, such as welding, from falling ill.

The Health Security Agency (UKHSA) raised the heat health warning to Level 4 for England for Monday and Tuesday.

Britain's Meteorological Office defines a Level 4 alert as a national emergency, to be used when a heat wave "is so severe and/or prolonged that its effects extend outside the health and social care system. At this level, illness and death may occur among the fit and healthy, and not just in high-risk groups."

The Met Office said "substantial" changes in working practices and daily routines would be required, and there was a high risk of failure of heat-sensitive systems and equipment, potentially leading to localized loss of power, water or mobile phone services.

Malthouse said the government was prepared for the extreme weather and would seek to learn lessons from it.

"We definitely need to adapt the way we build buildings, the way we operate and look at some of our infrastructure in the light of what seems to be an increasing frequency of these kinds of events," he said.

Heat wave engulfs Europe

The heat wave, 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 heat waves and drought.

In France's Landes forest, in the southwest Aquitaine region, temperatures "will be above 42 degrees Celsius" (107 degrees 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 40 degrees Celsius, 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.

In Spain, 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.

'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.

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 Agence France-Presse (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 47 degrees Celsius – a record for the month of July – on Thursday.