Temperatures have been soaring across Wales, with 30°C recorded by early afternoon.
The mercury has soared to 36.9°C in some spots on Thursday, driven by hot air funnelled from the south as western Europe is gripped by an extreme heatwave.
And a new record temperature for July has been set after the mercury reached 36.9°C at Heathrow Airport. The previous record was 36.7°C.
The majority of places in Wales were mid to high 20s, with 29°C recorded at the Royal Welsh Show near Builth Wells, and 30°C at Hawarden in Flintshire.
By 1pm, temperatures were pushing close to 36°C in a number of areas of the UK, including Cambridge, Northolt and Heathrow, the Met Office said.
Experts at the Met Office say the current weather pattern is driving hot air from the south, but there is "no doubt"' climate change is playing a role in driving what could be unprecedented temperature highs.
Temperatures were already into the 30°Cs by mid morning, with Kew Gardens seeing 34°C by 11am.
People are being urged to take precautions against the heat, including staying hydrated, staying inside at the hottest time of the day, avoiding exercise and wearing loose, light clothing.
Medical experts are warning that few lessons have been learned from last year's heatwave.
Network Rail announced that speed restrictions would be in place in some areas from midday until 8pm amid fears that tracks could buckle in the heat if trains travel too fast.
Speed limits on most commuter lines will be cut from 60mph to 30mph.
The Met Office said high pressure over eastern Europe and Scandinavia, combined with the position of the jet stream was funnelling hot air from Europe which had originated in north Africa.
But the kind of heatwave the country is experiencing is being made more likely, and more intense, by climate change, experts warn.
A study from the Met Office previously showed last year's summer heatwave was made around 30 times more likely than it would be under natural conditions as a result of human activity driving global warming.
