Forty people have drowned while swimming in unsupervised areas across France in recent days, the prime minister has said, as people across the country sought respite from the record-shattering heatwave sweeping across much of Europe.

“There is a tragic scourge of drownings,” Sébastien Lecornu said on Tuesday. “The latest figures we’ve received are 40 deaths since 18 June. Most of the victims are young people.”

Lecornu was preparing to chair a crisis meeting with ministers to address the ferocious early summer heatwave that has left parts of western France bracing for temperatures of up to 43C.

“We’re experiencing an episode of exceptional intensity,” Lecornu said. “Every day and every night, local and national temperature records are being broken.”

The national weather service, Meteo France, said 54 departments had been placed under a red heatwave alert as “oppressive and exhausting” heat smothered about half of the country. It said overnight temperatures were the hottest since record-keeping began in 1947.

Early on Tuesday, France’s national heat index, an average of the day and night-time highs measured at 30 weather stations across France, reached a record 21.6C, according to preliminary figures. The previous record of 21.4C was set on 25 July 2019.

Officials in the greater Paris region advised people to work from home as much as possible and avoid rail journeys. “The transport network comes under severe strain in periods of extreme heat … railways cannot withstand temperatures above 50 degrees,” the head of the Île-de-France region, Valérie Pécresse, told journalists.

The heat, which on Monday forced the closure of about 1,350 schools and was believed to be linked to the deaths of two young children in their family car, is forecast to continue until the end of the week.

“Further record-breaking temperatures are expected, including some that could surpass all previous records, regardless of the time of year,” Meteo France said.

The sweltering temperatures extending across swathes of Europe are caused by what Clair Barnes, a climate scientist at Imperial College London, described as a bulging mass of hot air.

“It’s drawing warm air up from north Africa, from the Sahara, and that’s why we have this really intense heat,” Barnes told Reuters. “It’s very slow moving and it means there’s kind of no wind, ‌no breeze for respite.”

In England, some schools closed early on Tuesday as the UK braced for the heatwave to set new records. With temperatures expected to soar to 40C, the Met Office issued its second ever red heat warning.

The UN secretary general, António Guterres, said in an address to a London Climate Action Week event on Tuesday: “London isn’t just calling. It’s cooking.”

He urged the world to work towards limiting global warming. “A climate crisis is pushing us deeper towards higher temperatures and closer to catastrophic tipping points, and an energy crisis is exposing the folly of a world hooked on hydrocarbons,” he said. “On the surface, these crises may seem separate, but they share the same destructive origin: fossil fuels.”

In Italy, the health minister declared a red heatwave alert in 15 cities including Milan and Rome. In Germany, officials said swimming accidents had spiked over the weekend, leading to the deaths of five people.

Nearly all of Spain was under a heat alert on Tuesday, with red alerts warning of “extraordinary danger” issued for areas around the southern city of Córdoba, the northern city of Bilbao and parts of the northern region of Cantabria.

On Monday, 101 of the 828 weather stations across Spain recorded temperatures of 40C or higher. At about 30 stations, temperatures remained above 25C overnight into Tuesday, underlining the intensity of the heatwave.

The situation was more severe in the south-eastern province of Almería, where temperatures had not dipped below 30C overnight for three consecutive night.

The barrage of heat dominated local headlines. “More than 72 hours above 30 degrees Celsius,” noted La Voz de Almería. In another article, the newspaper highlighted the consequences: “Almería doesn’t sleep: a hellish night of temperatures above 30C and highs exceeding 40C.”