This summer's fourth heatwave was expected to start by Wednesday, bringing temperatures in the area to 40C.
Similar temperatures are expected in Spain.
Dozens of communities are still threatened by wildfires that forced more than 70,000 to evacuate their homes over the last week.
Spain has ended some evacuation orders, allowing about 44,000 people to return home or leave confinement.
The Madrid region has been especially affected, but Prime Minister Pedro Sánchez said while there were highly challenging hours and days ahead, light was beginning to appear "at the end of the tunnel".
In neighbouring Portugal, 400 firefighters were tackling a large fire on three active fronts in the eastern Guarda area.
The French city of Bordeaux is not currently under threat, according to officials, despite the fact that one of the wildfires burning on Monday came only 15km (9 miles) from the city's periphery.
But Commander Rémi Lassoureille, who is leading the firefighting response in the Gironde region, is still worried.
"We are frightened about the impact on Bordeaux and the metropolitan area," he said. Air quality in the city also deteriorated greatly.
"By Sunday evening everything had really begun to empty out," a holidaymaker from Essex told the BBC, adding that the air was "really smoky" with bits of ash. "People were staying indoors, so it was eerie, and very sad."