In France, which is enduring its worst drought on record, flames raged through pine forests overnight, illuminating the sky with an intense orange light in the Gironde region, which was already ravaged by flames last month, and in neighbouring Landes. More than 68 square kilometres (26 square miles) have burned since Tuesday.

The French wildfires have already forced the evacuation of about 10,000 people and destroyed at least 16 houses.

Piotr Nieznanski, the conservation policy director at WWF Poland, said it appears that a toxic chemical was released into the water by an industry and the low water levels caused by the drought has made conditions far more dangerous for the fish.

“A tragic event is happening along the Oder River, an international river, and there is no transparent information about what is going on,” he said, calling on government authorities to investigate.

People living along the river have been warned not to swim in the water or even touch it. Poland’s state water management body said the drought and high temperatures can cause even small amounts of pollution to lead to an ecological disaster but it has not identified the source of the pollution.

The water level along Germany’s Rhine River was at risk of falling so low that it could become difficult to transport goods – including critical energy items like coal and petrol.

A national park in Portugal’s highest hill range, the Serra da Estrela, was also being ravaged by a wildfire. Some 1,500 firefighters, 476 vehicles and 12 aircraft were deployed to fight it but the wind-driven blaze 250 kilometres (150 miles) northeast of Lisbon was very hard to reach, with inaccessible peaks almost 2,000 metres (6,560 feet) high and deep ravines. The fire has charred 10,000 hectares (25,000 acres) of woodland.

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Heatwaves and wildfires engulf parts of US and western Europe

Heatwaves and wildfires engulf parts of US and western Europe

In Britain, where temperatures hit a record 40.3 degrees Celsius (104.5 degrees Fahrenheit) in July, the weather office has issued a new warning for “extreme heat” from Thursday through Sunday, with temperatures forecast to reach 36 C (96.8F).

It has been one of the driest summers on record in southern Britain, and the Met Office weather service said there is an “exceptional risk” of wildfires over the next few days.

London Fire Brigade said its control room had dealt with 340 grass, garbage and open-land fires during the first week of August, eight times the number from last year. Assistant Commissioner Jonathan Smith said “the grass in London is tinderbox dry and the smallest of sparks can start a blaze which could cause devastation.”

In Switzerland, a drought and high temperatures have endangered fish populations and authorities have begun moving fish out of some creeks that were running dry.

UN warns heatwaves will happen more often until 2060s

In Hausen, in the canton of Zurich, officials caught hundreds of fish, many of them brown trout, in the almost dried-up Heischerbach, Juchbach and Muehlebach creeks this week by anaesthetising them with electric shocks and then immediately placing them in a water tank enriched with oxygen, local media reported. Later, the fish were taken to creeks that still carry enough water.

Spanish state television showed dozens of trucks heading to France having to turn around and stay in Spain because wildfires had forced authorities to close some border crossings. TVE reported that truckers, many carrying perishable goods, were looking for ways to cross the border because the parking areas around the Irun crossing were full.

France this week is in its fourth heatwave of the year as it faces what the government describes as the country’s worst drought on record. Temperatures were expected to reach 40C (104F) on Thursday.