Both heatwaves were caused by a “heat dome” – a stalled area of high pressure that trapped hot air over the region, say experts.
The heat dome was made worse by human-induced climate change, the researchers say. This has warmed the planet by around 1.4C since pre-industrial times and scientists believe it added between 3 and 4C to the maximum temperatures recorded in May and June.
Hot tropical nights added to the sweltering conditions, offering little respite.
Many UK homes are not built to cope, leaving people vulnerable to prolonged, high temperatures.
Heat puts immense physical strain on the body, made worse if you are dehydrated, with the heart pumping harder and faster to cool you off.
Babies and older people are among those most at risk of harm, as well as those with existing health conditions.
It can lead to more heart attacks, strokes and other potentially fatal emergencies.
Intense heat can affect anyone, including fit and healthy people, and is dubbed a ‘silent killer’ because early symptoms are easily overlooked.
And when the hot air is very wet or humid, like it was in June, it’s more difficult for the body to cool down through sweating.
Prof Fredi Otto, an expert in climate science at Imperial who was involved in the research, told the BBC’s Today Programme: “Don’t underestimate the risks. Just because you’re fit and healthy, you’re not safe.”