Millions of people across Europe -- particularly in France, Italy, Spain and the UK -- sweltered through another wave of extreme temperatures Thursday as the continent's latest heatwave intensified.

Weather agencies recorded temperatures of more than 35C in several European countries, where more than 135 million people had been forecast to endure such weather.

"The heat has been unbearable here, honestly," said pharmacist Bailey Williamson, voicing concern for hospital patients as the UK saw its fifth-hottest day on record.

Europe is the planet's fastest-warming continent and in some places is ill-equipped to deal with hot weather.

Repeated heatwaves and drought have fuelled a historically hot summer of wildfires, and contributed to the deaths of thousands of people so far. 

Scientists blame ever hotter and more frequent heatwaves on human-caused climate change.

Temperatures were set to surpass 30C for nearly three out of five European residents (excluding Turkey) -- around 340 million people -- according to an analysis based on German weather service forecasts and 2025 population projections from the Joint Research Centre.

AFP's findings are consistent with those of Austrian NGO Klimadashboard.

Patients suffering in UK

In London, temperatures hit 38.1C at Kew Gardens, making it the fifth-hottest day on record for the UK, according to official weather agency the Met Office.

Williamson, 20, who works for the National Health Service (NHS), said the heat was hitting patients in hospital hard.

"People in beds are really, really sick, and they can't even breathe because of the heat problem -- so it's really bad," he said.

Hospitals had recorded an increase in heat stroke cases, and the NHS was struggling to keep up, he added.

"It's scary, it's scary," Williamson said of the summer's heatwaves. "I think it's a real sign about how bad climate change has really got."

Tim Wallace, 60, visiting from Australia, was also struck by the heat.

"I've never seen it like this, and it's clearly, it is concerning," he said.

"We might have summers like this more often, and the grass will burn off more often, and it'll come back. But it's all part of the planet getting hotter."

Europe swelters

In France, the weather service France Meteo recorded 38C in Paris and much of the country has been put on heatwave alert.

In Italy, similar temperatures were forecast to affect 29 million inhabitants, particularly along the west coast, in the Po Valley, the "heel" of the boot, and on the islands.

Large parts of the Iberian Peninsula were also likely to be affected, with 25 million people impacted in Spain.

Spain's Aemet weather agency recorded temperatures as high as 37C in the centre of the country Thursday.

In the UK, temperatures also hit 36C or higher in a total of 40 locations across central and southern England. 

The country is on track to record its hottest ever summer, the Met Office said on Tuesday. 

AFP calculates the figures using a method similar to that of Klimadashboard, cross-referencing population density with the weather forecast model from the Deutscher Wetterdienst (DWD) -- Germany's national meteorological service -- published at 0300 GMT.

Residents are counted if the model forecasts temperatures above 30C or 35C at their location at any point during the day.

As the model has a resolution of approximately 6.5 km it does not fully capture urban heat-island effects, when cities are significantly warmer than surrounding rural areas, David Jablonski of NGO Klimadashboard told AFP.

Consequently, the analysis "likely underestimates the number of people affected in densely populated urban areas", the organisation notes on its "European Heat Tracker" website.