France was in “a race against time” to control wildfires before another heatwave this week, fire service spokesperson Eric Brocardi told The Guardian.

Weather forecasters in both countries have said temperatures could reach 40C (104F) again is some parts from Tuesday.

Spain’s state meteorological agency says the heatwave will last until “at least” Sunday, “making firefighting efforts more difficult due to high temperatures [and] locally gusty southerly winds”.

Spain’s interior minister, Fernando Grande-Marlaska, said Monday and Tuesday were “absolutely crucial days” before temperatures rise again from Wednesday.

Prime minister Pedro Sánchez warned: “We face difficult hours, complex hours” ahead.

Experts have warned that the blaze raging in France and Spain has been driven by a “fire cloud”, which occurs when a wildfire generates its own unique weather system.

Known technically as pyrocumulonimbus, or “pyroCB” clouds, “fire clouds” are a unique phenomenon where a full thunderstorm forms within a fire plume.

They begin when intense fires release enormous amounts of heat and energy, driving huge amounts of hot air, smoke, and ash rapidly upward.

As this plume reaches high altitudes, low atmospheric pressure causes the rising air to expand and cool.

This rapid drop in temperature forces the moisture within the plume to condense into massive mushroom-shaped storm clouds.

Dubbed the “fire-breathing dragon of clouds” by NASA, these clouds can generate catastrophic conditions that accelerate the spread of further wildfires.

The Independent’s James C. Reynolds reports from Bordeaux:

British travellers arriving in Bordeaux on Monday braved the smoke as an “unprecedented” wildfire continued to sweep through southwest France, forcing more than 200,000 people to evacuate.

Steve Bray, arriving from London Gatwick, told The Independent that he and his partner, Laura Slade, were spending a couple of nights in the city before heading to wedding in the south.

“Our government didn’t say anything, the French government didn’t say anything, the flights didn’t say anything and the hotel didn’t say anything,” he said.

They assessed that “the French authorities are on top of it and if it was ‘bad bad bad’, then we wouldn’t have been allowed to fly in the first place”.

“It’s all good. Wasn’t put off at all,” he said, adding: “Overall, can’t complain. We’re on holiday, so – haven’t got work tomorrow!”

As millions are affected by uncontrollable wildfires raging across Europe, and Britain braces for its fourth heatwave of the year, it is time we all accepted the evidence of our own eyes:

The mayor of a town near Bordeaux has described the current blaze and the “fire of the century”.

Jerome Steffe, from the town of Cestas, told Sky News that 17,000 residents had to be evacuated in previous days.

“This fire is so enormous… it’s something we have never, never seen in France. I think this is the fire of the century.”

Mr Steffe told the broadcaster that the fires had been stabilised on Monday thanks to the efforts from fire crews and lower temperatures and some rainfall.

He said his town was bracing for temperatures that could clim above 41C.