Wednesday was the hottest June day in more than 40 years, the Met Office announced.
A temperature of 33.9C was recorded at Heathrow airport, the authority said, making it the hottest June day since 1976.
Temperatures are at their highest for a June day since the 35.6C (96F) recorded in Southampton on 28 June 1976, and it is the warmest summer solstice on record.
Roads have melted and officials have issued a heatwave warning as Britain basks in its longest period of very hot weather for two decades.
The mercury has been above 30C for five days in a row for the first time since 1995.
But there are also weather warnings in place for Wednesday afternoon and evening, with heavy rain and thunderstorms forecast for parts of southern Scotland, northern England, north Wales and the Midlands
Dozens of people have been treated by paramedics on the hottest ever day of Glastonbury Festival.
Temperatures at the 900-acre Somerset site reached 30C by 2pm on Wednesday and were expected to rise further during the afternoon.
Before Wednesday, the hottest days at the festival were in 2010 and 1989, when temperatures reached 27.3C.
In those years, revellers were left with heat stroke and exhaustion.
A spokesman for South Western Ambulance Service said 38 people had been treated by paramedics by 2.30pm on Wednesday.
"This is for a range of incidents and not just related to the heat," he added.
Additional reporting by agencies
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