The UK is basking in its hottest June day in 41 years, with a temperature of 34.5C (94F) recorded at Heathrow.
The Met Office reading at the London airport is the highest in June since the mercury hit 35.6 (96F) in 1976 - the all-time high since records began.
The heatwave has seen five sizzling days in a row during which temperatures in parts of the UK have topped 30C.
But weather warnings have also been issued for rain, with thunderstorms expected in some areas.
BBC weather presenter Chris Fawkes said: "To have these really prolonged spells, you need a block of high pressure that directs other weather fronts away.
Then we get the hot weather coming up from Europe."
Storms are forecast for some areas later in the day, and yellow weather warnings for rain in parts of England, Wales and Scotland are in place until the early hours of Thursday.
Our presenter added: "It is all going to go bang tonight.
"The hot air from the surface will meet with colder air coming in from the Atlantic and we will have some big thunderstorms, gusty winds, heavy rain and, in some places, even hail.
Summer of 1976
The record-breaking summer of 1976 saw nine weeks of blazing sunshine.
Between June and August, blue skies were a daily occurrence and, for two weeks, temperatures were 32C or above consistently.
Five days saw temperatures exceed 35C.
The hottest day of all was 3 July, with temperatures hitting 35.9C in Cheltenham.
A downside was the worst drought in recorded history for the UK, building up from a warm summer in 1975 and incredibly dry months after.
Rainfall in the winter of 1975 was half of what it normally would be, and by the summer of 1976 crops were affected and there were forest fires in the south of England.
But once the summer had come to a close, autumn saw intense rainfall and natural balance was restored.
The crowd heading to Royal Ascot were hoping for a similar relaxation of dress code to the one race-goers experienced on Tuesday.
Normally men attending the horse racing event in the Royal Enclosure must wear black or grey morning dress with a waistcoat, tie and a top hat.
But the jacket rule was not enforced for the first time in its history, as organisers took a "common sense approach".
Whilst some are taking to the parks or the beach to lap up the sunshine, people on the daily commute have been faced with travel disruption.
Greater Anglia Trains has cancelled a number of its services between London and Essex, saying the heat has led to speed restrictions on the lines to stop them from buckling.
For those in more congested areas, the Department for Environment and Rural Affairs has also warned there may be pockets of high pollution during the exceptionally hot weather, especially in the East Midlands.
Emergency services have warned people to keep safe in open water after three people died in separate incidents around the country.
A 15-year-old boy died after getting into difficulty when swimming in a lake with friends in Walsall, West Midlands.
This follows the deaths of a teenager who drowned in a reservoir near Rochdale and an elderly woman who rescue teams tried to save off the coast of Sussex.
A West Midlands Ambulance Service spokesman said: "Although it is very hot at the moment, the dangers of going into open water cannot be underestimated."
Tips for coping in hot weather
- Shut windows and pull down the shades when it is hotter outside. You can open the windows for ventilation when it is cooler.
- Stay out of the sun and don't go out between 11:00 and 15:00 (the hottest part of the day) if you're vulnerable to the effects of heat.
- Have cool baths or showers, and splash yourself with cool water.
- Drink cold drinks regularly, such as water and diluted fruit juice. Avoid excess alcohol, caffeine (tea, coffee and cola) or drinks high in sugar.
- Wear loose, cool clothing, and a hat and sunglasses if you go outdoors.
- Check up on friends, relatives and neighbours who may be less able to look after themselves.
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