Europe is getting hotter, with London bracing for Britain's hottest June day since 1976 as Portugal battled to stamp out deadly forest fires.

More than 1,000 firefighters were still fighting to control the flames that broke out in central Portugal at the weekend, killing 64 people.

Cooler weather was aiding their efforts, but thermometers were still hovering around 35 degrees Celsius (95 degrees Fahrenheit) -- a level matched across oven-like swathes of Europe, including Italy, Austria, the Netherlands and even alpine Switzerland.

Two forest fires have also broken out since Sunday on Croatia's southern Adriatic coast, prompting the authorities to evacuate 800 tourists, though the blazes have now been brought under control.

"We were scared, it's true," a Swedish tourist who gave her name only as Karolina told local media. "From the hotel room window it seemed like the fire was at our doors."

As the northern hemisphere marked the summer solstice, firefighters were on alert in Austria, where revellers were due to celebrate the longest day of the year by lighting bonfires for the festival of Saint John.

In Italy, forecasters say the current heatwave could turn out to be the most intense in 15 years, with temperatures around eight degrees above the seasonal average -- 39 degrees Celsius in Milan and up to 30 in the Alps at an altitude of 1,000 metres (3,300 feet).

A study released Monday warned that deadly heatwaves will become more and more common around the world even if the rise in temperatures is capped at 2.0 degrees Celsius as targeted by the Paris Agreement to combat climate change.