Summer heat waves in France, Germany, Spain and Britain led to more than 20,000 "excess" deaths, a report compiling official figures said on Thursday.

Temperatures hit nearly 40 degrees Celsius or above from Paris to London in 2022 and climate scientists from the World Weather Attribution group found that such high temperatures would have been "virtually impossible" without climate change.

A heat wave in 2003 caused more than 70,000 excess deaths across Europe, largely in France, and led many countries to implement measures such as early warning systems, asking people to check on others and opening air-conditioned schools.

This and related action plans may have eased some of the impact of heat waves in 2022, but the death toll was still "higher than expected," said Chloe Brimicombe, a heat waves researcher at the University of Graz in Austria.

"I consider this ... the most impactful heat wave since 2003," she said.

Because authorities do not attribute most deaths directly to heat, statisticians use the excess formula to give an estimate, looking at how many more people died in a given period than would be expected compared with a historical baseline.

Heat can kill by inducing heatstroke, which damages the brain, kidneys and other organs, but it can also trigger other conditions such as a heart attack or breathing problems.

The World Meteorological Organization said this month that Europe had warmed more than twice as much as the rest of the world over the past three decades, while the Copernicus Climate Change Service said summer 2022 was the hottest on record.

France reported about half of the summer's excess deaths in Western Europe, with 10,420 fatalities in total.

Excess deaths reached 3,271 in England and Wales during the summer, Britain's Office of National Statistics reported.

Spain recorded 4,655 heat-attributable deaths between June and August while the German health agency reported 4,500.