Brutal heat torching southern Europe has forced more than 20,000 people to flee wildfires in France as a marine heatwave scalds the Mediterranean and researchers warn that “Dantesque” scenarios cannot be ruled out as infernos worsen.

A “particularly intense” wildfire on France’s Atlantic coast prompted a series of evacuations, including thousands of tourists from campsites and other holiday accommodation, as firefighters struggled to control the blaze.

Extreme highs above 44C were expected in Spain on Thursday, and above 38C on the Italian islands of Sardinia and Sicily. The previous day, the Spanish cities of Albacete, Ávila and Teruel broke their July temperatures records with heat as high as 42C, according to the Spanish meteorological agency, Aemet.

A marine heatwave is affecting 80% of the Mediterranean Sea’s surface area, according to the Mediterranean Monitoring System. Daily temperatures on Tuesday had reached “extreme” levels in several parts, with some anomalies 5C hotter than the average from 1991-2020.

People look at the sky illuminated by wildfires in Le Porge, south-west France, on Thursday. Photograph: Romain Perrocheau/AFP/Getty

A man rests with his dog in a community hall on a camp bed in Le Porge. Photograph: Romain Perrocheau/AFP/Getty

The severity of fires in the EU has broken records for this time of year, data from the European Forest Fire Information System showed on Wednesday, and about twice as much land had burned by 22 July than the average for that date over the last two decades. France has broken records for the area burned, and both it and Spain have broken records for the number of fires.

The blaze in France, which began in a dense pine forest near the village of Saumos about 40km (25 miles) west of Bordeaux and just north of the bay of Arcachon, a popular tourist destination, had burned more than 3,400 hectares (8,401 acres) in less than 24 hours, firefighters said on Thursday.

No casualties had been reported and only one home destroyed so far, they said, but the Gironde prefecture said that while 700 firefighters and four water-bombing planes were battling to bring the fire under control, the situation remained “unfavourable” and the blaze was not yet stabilised.

France has endured three almost back-to-back heatwaves since May as climate breakdown leads to more frequent and intense extreme weather events causing thousands of excess deaths, evaporating soils and waterways and fuelling wildfires.

Smoke rising from the wildfire in Saumos is photographed from Arès on Wednesday. Photograph: Romain Perrocheau/AFP/Getty

Patrick Martineau, 69, who was evacuated from the village of Le Porge, told Agence France-Presse that police had “come banging on the doors … It’s scary – hard to take in. When we bought a home here, we never thought there’d be a fire risk.”

Sylvie, a holidaymaker from Paris staying in the relatively more built-up area of Lège-Cap-Ferret, said a neighbour had told her: “‘Get your bags ready, we’re going to be evacuated during the night.’ We put the bulk of our stuff in the car boot and left.”

Elke Urbain, from Antwerp in Belgium, who holidays in the area every year, said she was advised while she was on the beach on Wednesday afternoon to “go back immediately to the campsite and gather your things. It’s just terribly sad.”

In 2022, a wildfire burned more than 30,000 hectares in the same area, forcing about 50,000 people to be evacuated. In tinder-dry conditions and fanned by a strong wind, the latest blaze has torn through the forest, with flames reaching 10 metres high.

Jean-Luc Helias, 54, from Le Porge, said he and his family had lived through that fire four years ago and “the firefighters do an amazing job to protect people’s homes. All that matters to me is that my family is safe; the rest is just material possessions.”

Nine local municipalities have opened reception centres for evacuees and were sheltering more than 2,500 people, the prefecture said. About 8,800 people were evacuated at midday on Thursday, after 2,000 in the morning and 10,000 overnight.

Numerous roads in the area – including those leading to the Cap Ferret peninsula a few kilometres to the south, home to the villas of celebrities and millionaires and one of France’s most exclusive holiday spots – were closed to all but essential traffic.

A helicopter drops water over a burning area as a wildfire flares up again in Pontevès in south-eastern France on Wednesday. Photograph: Thibaud Moritz/AFP/Getty

Police have opened an investigation into the causes of the wildfire, which the mayor of Le Porge, Martial Zaninetti, said on Wednesday appeared to have been started by a machine that was clearing undergrowth.

A house in Cotignac, another village close to Pontevès, is consumed by fire on Wednesday night. Photograph: Thibaud Moritz/AFP/Getty

A fortnight ago, flames devastated a 10th of the Unesco-listed Fontainebleau forest outside Paris in a rare wildfire so far north. France’s interior minister, Laurent Nuñez, has said more than 12,500 wildfires had broken out since the beginning of the year.

In Spain, most of the people evacuated on Wednesday evening owing to a fire 70km south-west of Madrid were also allowed to return home. A fire that has burned 32,000 hectares in the province of Guadalajara, north of the capital, was also described as stable.

Spanish wildfire scientists said they were not surprised by the extreme figures as the abandonment of rural land had left more overgrown vegetation and the rise in temperatures from climate breakdown made it easier for such plant matter to dry out and burn.

Víctor Resco de Dios, a professor of forestry engineering and global change at the University of Lleida, said: “Although it’s difficult to predict how far things might go, we will soon be unable to rule out Dantesque scenarios like those seen in California or Australia, with fires or fire complexes reaching hundreds of thousands of hectares or even a million hectares.”

The marine heatwave in the Mediterranean is projected to shrink but stay strong from Sardinia to Greece, according to a forecast for Saturday from Mercator Ocean. From Gibraltar to the Balearic Islands, strong heat is expected to drop to moderate levels, despite intensifying locally in the Alboran Sea.

Western Europe has been rocked by repeated heatwaves this summer, with extreme highs in June that scientists expect will have killed tens of thousands of people as hot days push bodies into overdrive and warm nights prevent them from resting.

On Wednesday, an analysis estimated the cost of the June heatwave to the continent’s grain farmers at €2bn (£1.7bn) in lost revenue from 9m tonnes of destroyed crops. On Thursday, scientists found climate breakdown had contributed to drought conditions, making levels of soil dryness five times more likely in western Europe and 11 times more likely in eastern Europe.