A record high temperature of 40 degrees C has provisionally been exceeded in the UK for the first time ever, Britain's Met Office said on Tuesday, shortly after it reported a record of 39.1 degrees celcius in southern England.

"London Heathrow reported a temperature of 40.2°C at 12:50 today. Temperatures are still climbing in many places, so remember to stay #WeatherAware," it said on Twitter. 

The London Fire Brigade has declared a major incident in response to the surge in fires across the British capital, Mayor Sadiq Khan said on Tuesday.

"This is critical: @LondonFire is under immense pressure. Please be safe," Khan said on Twitter.

Other European countries, including Belgium, the Netherlands and Germany are also experiencing a heatwave, while wildfires still threaten parts of southwest France and Spain.

The World Meteorological Organization (WMO) said on Tuesday that Europe's heatwave looks set to peak on Tuesday, but temperatures may remain above normal into the middle of next week.

"The direction is clear and in the future these kind of heatwaves are going to be normal and we will see even stronger extremes," said WMO Secretary-General Petteri Taalas.

(With input from Reuters)