More than 350 French towns have recorded their highest-ever temperatures for May as France and the UK set national heat records amid an extreme early-summer heat event that could see the mercury rise to 40C in parts of Spain by the end of the week.

The UK’s Met Office said the country’s all-time record for May was broken when a temperature of 34.8C was recorded at London’s Kew Gardens.

Météo France said late on Monday that new monthly highs for May had been recorded at 352 weather stations mainly in western France, with the highest – 37.1C – registered near Hossegor, in the south-western department of Landes.

“This is an unprecedented event with a one in 1,000 chance of happening at this time of year based on the climate from 1979 to 2025 and virtually impossible in the preindustrial era,” Christophe Cassou, a climate scientist, told Le Monde.

More new highs are likely to be set in France, Spain and the UK, forecasters said, with temperatures exceeding norms by 12C or 13C in what Météo France described as a “premature, remarkable and long” heat episode expected to last several more days.

France’s national weather agency said the record temperatures were caused by a heat dome, with hot air from Morocco trapped under an area of high pressure, adding that Europe could expect such events to “occur more and more often and earlier and earlier, and to be more and more intense”.

Models have already estimated that, with the effects of climate breakdown, June heatwaves are now about 10 times more likely in Europe than they were in the preindustrial era, and the same trajectory is becoming visible for May.

“This extension of the heatwave season is entirely characteristic of the effects of climate change,” Robert Vautard, a climate researcher, told Agence France-Presse. “Eventually, we will be seeing similar heat events in April and October.”

Thirty-one of metropolitan France’s 96 administrative departments have been placed on high-temperature alert until Tuesday, including eight on level orange, the second-highest, requiring residents to “take precautions”. It was first time the country’s national heat warning system has been activated during May since it was introduced in 2004.

Météo-France said in a bulletin on Monday that temperatures could climb locally to near 36C in multiple towns and cities, and to 37C on Tuesday. “The west of the country will see temperatures several degrees higher than ever recorded in May,” it said.

The mercury rose past 35C in the western towns of Niort and Nantes and reached 34.3C in Poitiers, while the capital, Paris, approached 33C. Much of the north-western region of Brittany was expecting temperatures of between 33C and 35C on Tuesday.

Le Parisien newspaper said the national temperature average, measured at 30 weather stations across the country, hit a record 24.4C on Monday, against a previous high of 23.7C dating back to 1944. The figure has not yet been confirmed by Météo France.

A man died during a 10km running race in the Paris suburb of Maisons-Alfort on Sunday, civil defence services said, reportedly after suffering a heart attack, while 10 more runners had to be taken to hospital in critical condition after the race.

The hot spell in Spain – where temperatures in some southern areas hit 38C over the weekend, between 5C and 10C higher than normal – is also expected to continue through the week, said Rubén del Campo of the state meteorological office Aemet.

“The other really notable thing is that the situation is going to last until at least the end of the week. In fact, it could get even hotter on Thursday and Friday, with temperatures of at least 34C across most of the country,” del Campo said.

Widespread highs of 36-38C in the Guadiana, Guadalquivir and Ebro valleys are expected between Wednesday and Friday, he added, saying that “in some of those areas, temperatures could reach 40C”.

Del Campo also said much of the country could expect so-called “tropical nights”, in which the night-time temperature does not drop below 20C.

Parts of the UK could enter a heatwave, with temperatures exceeding 26C to 28C – depending on the location – for three days. In France, night-time temperatures must also stay above a certain level for an official heatwave to be declared.