The UN climate chief has said an extreme early heat event sweeping parts of western Europe was “a brutal reminder of the spiralling impacts of the climate crisis”, after France and the UK set new temperature records for May on two consecutive days.

Simon Stiell, the executive secretary of the UN Framework Convention on Climate Change, said on Wednesday the “main culprit” was humanity’s burning of coal, oil and gas – known to be the primary driver of climate change.

“The science is clear that human-induced climate change is making these heatwaves more frequent and extreme,” Stiell said, as France, Spain and the UK sweltered in temperatures usually associated with July or August.

“Protecting human lives, businesses and economies from extreme heat and the many other soaring costs of climate change is core business for every nation, and it starts with kicking the fossil fuel addiction much faster.”

The war in the Middle East had laid bare the “soaring costs” of fossil fuel reliance and the need to pivot to cleaner sources of energy, Stiell said, also noting 43C-plus conditions in India, where authorities have reported deaths from heatstroke.

A temperature of 35.1C (95.2F) was recorded at Kew Gardens in London on Tuesday, the UK’s Met Office said, breaking the 34.8C record set a day earlier. The readings easily surpassed a previous record of 32.8C that was set in 1922 and equalled in 1944.

France, which was expecting local highs of 39C on Wednesday, also recorded its hottest May day ever on Tuesday, when the national heat index, an average of 30 readings around the country, hit 24.8C, surpassing Monday’s 24.6C – itself a record.

Météo-France, the national weather service, said a “heat dome” – with heat held in place by a high-pressure weather front – was producing temperatures up to 13C higher than customary for the time of year.

Seventeen of France’s 96 administrative departments, including Paris, were placed on an orange high temperature alert for Thursday, the second highest level, requiring the population to “be very vigilant and take precautions”. Another 29 were on a more moderate yellow warning.

Scientists have said that as the Earth warms, extreme heat events historically confined to high summer were becoming more frequent and more intense, as well as happening earlier and later in the year, putting more people in danger.

“We know beyond a shadow of a doubt that events such as this have been made more likely and more severe due to climate change arising from our emissions of heat-trapping greenhouse gases,” said Peter Thorne, the director of climate research at Maynooth University in Ireland – where a record May high of 28.8C was recorded on Monday. “Nevertheless, many of the records being set, particularly in the UK and France, are mind-bogglingly crazy.”

French authorities on Tuesday reported at least seven deaths directly and indirectly linked to the high temperatures – two of competitors taking part in sporting events, and five drownings as many people sought relief at swimming spots. Authorities in Britain said four teenagers have drowned in England since Sunday.

In Spain, where temperatures could reach 40C this week, an orange weather alert was issued for the Basque Country amid predictions the northern region could reach 37C on Wednesday. Temperatures of 36C to 38C were forecast for southern regions, with a high of 38C in the south-western city of Badajoz.

Spain’s state meteorological office, Aemet, said temperatures more normally seen in July had already been recorded across the country, and the heat was “more characteristic of the dog days, the hottest period of the year”.

Rubén del Campo, an Aemet spokesperson, added: “Both this episode and the atmospheric pattern that’s causing it are part and parcel of climate change and of what’s been observed in recent years.”