Dramatic warning: 4,500 people are expected to die in three days; The country bordering Serbia has been warned

The World Health Organization (WHO) issued a dramatic warning yesterday calling for urgent action to prevent "tens of thousands of unnecessary and preventable deaths".

"It's no longer a question of whether we'll be hit by a heat wave, but how many of them we'll have this year and how long they'll last," said Marisol Iglesias Gonzalez, technical advisor on climate change and health at the WHO.

Statistician Pierre Masselot of the prestigious London School of Hygiene and Tropical Medicine told Politico that the current heat wave, between June 30 and July 3, could cause more than 4,500 excess deaths.

The countries likely to record the highest rates of excess mortality are Italy, Croatia, Slovenia and Luxembourg, he said. "The worst days will probably be Tuesday and Wednesday," Masselot added.

The WHO estimates that heatwaves claim more than 175,000 lives annually in the European region, which stretches from Iceland to Russia.

A large study co-authored by Masselot, published in January and covering 854 European cities, warns that heat-related deaths will rise sharply if countries do not start investing seriously in adapting to climate change.

The WHO reiterated that heat waves are a direct consequence of climate change fueled by the burning of fossil fuels, and that they will become more frequent, more dangerous and more intense, with a growing number of illnesses and deaths.

Spain already on the brink: 804 places under the highest level of warning

Almost two-thirds of cities in Spain are facing warnings of health risks, and as many as 804 are under the highest level of alert, according to data from the national meteorological agency AEMET (Spanish: Agencia Estatal de Meteorología).

Intense heat is expected across the country until July 3.

Also, heatstroke warnings were issued in France, Italy, Portugal and Greece.