A new provisional UK record temperature has been recorded as 40.3C at Coningsby in Lincolnshire, the Met Office has said.
Much of England and Wales was under a “red” extreme heat alert until the end of Tuesday with warnings of the potential for serious illness and a danger to life.
In one of the most significant developments, more than 100 fire engines were despatched to multiple blazes across London as a major incident in the capital was declared.
The record was broken three times in a few hours with 39.1C measured in Charlwood, Surrey and 40.2C at Heathrow Airport - the first time since records began the temperature has topped 40C.
Experts said at least 29 observation sites across England had provisionally broken the previous UK record temperature of 38.7C, which was reached at Cambridge Botanic Garden on 25 July in 2019.
Scientists have warned that the extreme heat is fuelled by climate change, which is making heatwaves more intense and frequent.
People have been warned to stay indoors, drink fluids, and look out for those who are vulnerable or live alone.
In the capital, the London Fire Brigade (LFB) declared a "major incident in response to a huge surge in fires across the capital today”.
A spokesperson said that the service is under “immense pressure” as a result of the heat.
The LFB said they were in attendance of 10 fires across, London, many of them grass fires.
Over 100 firefighters were deployed to tackle a huge blaze in Wennington, east London, with pictures from the scene showing thick black smoke and charred buildings.
At least one home appeared to be completely destroyed in the blaze, while smoke covered a wide area.
Nearby fields were seen to be scorched.
The fire brigade said on Twitter: “We’ve got 15 fire engines and around 100 firefighters dealing with this blaze on The Green in #Wennington.”
The heat peaked at 38.1C in Santon Downham, Suffolk on Monday, making it the hottest day of the year and the third hottest day on record, after Cambridge in 2019 and 38.5C in Faversham, Kent, in August 2003.
Wales provisionally recorded its hottest ever day on Monday, too, with temperatures reaching 37.1C at Hawarden in the west of the country.
The previous record high was 35.2°C, recorded at Hawarden Bridge, Flintshire on 2 August, 1990.
While many flocked to beaches and rivers to make the most of the hot weather, health experts warned of the dangers of the high temperatures.
The UK Health Security Agency issued a heat health warning at level four, which is described as a “national emergency”.
A number of teenagers are believed to have died after jumping into lakes and rivers during the heatwaves.
Watch: New UK record as temperature hits 40.2C - with 41C expected later
In London, a 14-year-old boy is feared to have drowned after getting into the River Thames to cool off on Monday.
The Metropolitan Police said they were called just before 5pm to reports a child had entered the water at Tagg's Island near Hampton Court.
A search for him has so far not been successful.
In Berkshire, a body was found in the search for a teenager on Monday.
Thames Valley Police said they were called to Bray Lake in Berkshire at 11.45am on Monday morning.
A body was discovered at 1.30pm and confirmed dead at the scene.
A body was also found in the search for missing Robert Hattersley, 13, who went missing after jumping into the River Tyne near Ovingham, Northumberland, on Sunday.
Paying tribute, his family said “he brought a smile to so many people’s faces and he will be missed by absolutely everyone who knew and loved him”.
Search teams were sent out and on Monday morning Northumbria Police said the boy's body had been found following extensive searches by the force’s marine unit.
Rail services have been heavily disrupted on Tuesday, with no services into or out of London Kings Cross all day, no Thameslink or Great Northern trains north of London, and only very limited services on East Midlands Railway.
There are also very limited and disrupted services running into and out of London Euston, on Avanti West Coast and West Midlands Railway, and London Marylebone on Chiltern Railways, and temporary speed restrictions in the face of the risk of buckling rails.
Figures have shown that nine of the UK’s 12 hottest days on record since 1884 have occurred within the last two decades - with four out of five taking place in the last three years alone.
Only one of top 12 hottest days took place before 1990.
Met Office figures show that spikes in the UK’s daily maximum temperature are becoming both more frequent and more recent.
A similar trend is evident in figures showing the UK’s annual average maximum temperature.
Top temperatures recorded in the UK
19 July 2022, Coningsby Lincolnshire: 40.3C (provisional)
25 July 2019, Cambridge Botanic Gardens: 38.7C
10 August 2003, Faversham in Kent: 38.5C
18 July 2022, Santon Downham in Suffolk: 38.1C
31 July 2020, Heathrow Airport in London: 37.8C
3 August 1990, Cheltenham in Gloucestershire: 37.1C
1 July 2015, Heathrow Airport in London: 36.7C
9 August 1911, Raunds in Northamptonshire: 36.7C
2 August 1990, Worcester in Worcestershire: 36.6C
19 July 2006, Wisley in Surrey: 36.5C
7 August 2020, Kew Gardens & Heathrow Airport in London: 36.4C
6 August 2003, Gravesend in Kent, 36.4C