Southern and western Germany and Belgium were also braced for potentially record-breaking temperatures as the heatwave, which scientists attribute to climate change, edged north and east.
Numerous wildfires were reported in Italy. One of the biggest blazes broke out on Monday night in the hills of Massarosa in Tuscany, and was still raging on Tuesday afternoon.
"Fire continues to devour the woods in a frightening way due to the wind," Tuscany Governor Eugenio Giani said, noting that 365 hectares (900 acres) of land had been destroyed.
Fires were also reported in woods near Rome, as well as on the shores of Lake Orta north of Milan and near the northeastern city of Trieste.
A wildfire fuelled by strong winds raged on a mountainous area near homes on the outskirts of Athens, prompting authorities to order the evacuation of at least one area.
A temperature of more than 40C (104F) was provisionally recorded for the first time in Britain, the Met Office said, and authorities have put Britain on a state of "national emergency" over the unprecedented temperatures.
Train routes from London up the east and west coast of the country were cancelled and normally busy city centres appeared quiet. Network Rail tweeted pictures showing bends and kinks in the tracks.
To the east of the capital a large fire engulfed homes in the village of Wennington, with flames tearing across neighbouring fields and approaching a historic church. Large grass areas around the capital were on fire.
FIRES 'NOT STABILISED'
In southwestern France, the wine-growing Gironde region saw its biggest wildfires in over 30 years and authorities said a man had been detained on suspicion of arson.
The fires have spread across 19,300 hectares (about 75 square miles) in the countryside surrounding Bordeaux since July 12, forcing 34,000 people to evacuate their homes.
About 2,000 firefighters, supported by eight water-bomber aircraft, were battling the blazes.
With human-caused climate change triggering droughts, the number of extreme wildfires is expected to increase 30% within the next 28 years, according to a February 2022 United Nations report.
"We are seeing more frequent heatwaves, and the heatwaves are hotter than they would have been without climate change," Friederike Otto, senior lecturer in climate science at Imperial College London, told Reuters.
The health impact of the heatwave has been in focus, with particular care given to the elderly and vulnerable.
The head of Portugal's health authority DGS, Graça Freitas, told Reuters that 1,063 excess deaths due to the heatwave, above normal levels, were recorded from July 7 to 18.
"Portugal ... is among one of the areas of the globe that could be (more) affected by extreme heat," Freitas said. "We have to be more and more prepared for periods of high temperatures."
Carlos Antunes, a researcher at Lisbon University's faculty of sciences, said the data showed the elderly were most likely to die due to heatwaves.
Hot nighttime temperatures are also hindering firefighting responses across Europe and worsening health conditions as the night hours fail to provide a cooling reprieve, experts said on Tuesday.
FLAMES AND SMOKE
In Italy, temperatures were expected to hit 40C across a swathe of the north and centre this week, as well as the southern heel of Italy's boot, Puglia, and the islands of Sardinia and Sicily.
Five cities were placed on the highest red alert because of the heatwave on Tuesday. The alert, which warns of serious health risks tied to the weather, will cover nine cities on Wednesday, rising to 14 on Thursday, including many of Italy's largest metropolitan areas such as Rome, Milan and Florence.
Although the mercury dipped back towards more normal summer levels in Spain and Portugal, firefighters in both countries were still battling multiple blazes.
More than 30 wildfires continued to ravage parts of Spain, with authorities paying special attention to four blazes in Castile and Leon and Galicia.
In Losacio, in northwestern Zamora province, where two people have died and three were critically injured, more than 6,000 people in 32 villages have been evacuated.
So far this year 70,000 hectares (173,000 acres) have been burned in Spain, around twice the average of the last decade, official data showed before the heatwave.