Thermometers hit 40.3C (104.5F) at Coningsby in Lincolnshire, one of 34 locations to surpass the previous high of 38.7C – set in July 2019.
London Fire Brigade declared a major incident after a surge in fires, including grassland blazes, and power has been disrupted in parts of England.
Some rail services have been cancelled after tracks overheated or buckled and overhead cables failed.
Hundreds of firefighters have been tackling fires across London, while all trains were suspended between London Euston and Milton Keynes due to a lineside fire – caused when overhead electric cables came down in Harrow.
Rail operator c2c said it had closed the line between Ockendon and Upminster due to a fire near the line in east London, while the A40 was closed in both directions due to a fire at Northolt in west London.
There are fires elsewhere in the country and major incidents have also been declared in several areas including Leicestershire, South Yorkshire, Suffolk and Norfolk.
Flames tore through several homes and buildings in the village of Wennington on the outskirts of east London, while a nursery was caught in a blaze in Milton Keynes.
In the capital people have been asked not to have barbecues or bonfires this evening and to be careful when disposing of cigarettes.
Blazes are raging near the village of Wennington on the outskirts of London
Heathrow Airport was the first place to break the 40C mark, hitting 40.2C at 12:50 BST but as of 16:00 several other places had also passed 40C, including Gringley on the Hill in Nottinghamshire and St James’s Park, Kew Gardens and Northolt – all in London.
A large swathe of eastern England, from Surrey to South Yorkshire, saw temperatures between 39C and 40C, and at least 34 weather stations exceeded the previous UK temperature record.
Scotland has also seen a new record, according to provisional Met Office figures, with 34.8C recorded at Charterhall, in the Scottish borders – beating the previous record of 32.9C recorded in 2003.
In Wales, Hawarden in Flintshire registered 36.2C, provisionally the second highest temperature on record in Wales, but below Monday’s 37.1C.
The two days of extreme temperatures prompted the Met Office to issue a red extreme heat warning for the first time, which expires at midnight.
There have been warnings for people to be careful around water after at least five people died in and around water over the past two days.
The body of a teenage boy who went missing on Monday was recovered from the River Thames in Richmond, west London, on Tuesday afternoon, the Metropolitan Police said.
Amid the extreme heat Network Rail had warned against travel, road surfaces have warped, and a number of power cuts have been reported.
Rail services on Wednesday morning could continue to face some disruption as problems are fixed, Network Rail has said.
Met Office chief of science and technology Prof Stephen Belcher said exceeding 40C was “virtually impossible” in an undisrupted climate, but due to climate change “driven by greenhouse gasses” such extreme temperatures had become a reality.
He said if there continued to be high emissions “we could see temperatures like this every three years”.
In other developments:
• Five people were rescued and a sixth is missing after a search off Clacton Pier in Essex
• A man has died after being pulled from the sea on the Isle of Wight
• Sagging conductors and overheating transformers caused faults with the power network in Yorkshire, Lincolnshire and the North East
• Network Rail issued a “do-not-travel” warning for Tuesday, affecting services travelling through the “red zone”
• Thameslink, Great Northern, East Midlands and East Coast services are heavily disrupted – or cancelled altogether
• There are no services out of London King’s Cross, St Pancras International or London Euston
• The Supreme Court moved hearings online, while the British Museum confirmed it would shut early at 15:00.
The extreme warning, indicating a threat to life, covers an area stretching between London, Manchester and York.
There was a markedly cooler outlook for Penzance, Cornwall, where thundery storms hit the popular holiday destination, and a band of showery rain is expected to progress eastwards – bringing fresher temperatures.
And across the country, the heatwave is forecast to come to a sudden end with markedly cooler weather expected on Wednesday and thunderstorms forecast for parts of the east and north-east of England.
Provisional figures showed the UK experienced the warmest night on record from Monday into Tuesday.
Emley Moor in West Yorkshire is likely to have broken that record, where 25.9C was recorded overnight, according to the Met Office. The heat soared in the morning – with temperatures hitting a record 40.2C by 12:50
But showers in Cornwall dampened down the heat
Rail services have been heavily impacted by the extreme heat, Network Rail said, with buckled rails reported and overhead wire systems failing.
A record rail temperature of 62C was recorded in Suffolk.
Jake Kelly, the group director for system operation at Network Rail, said it had taken “the difficult and regrettable” decision to close the East Coast Mainline and the Midland Mainline on Tuesday due to record temperatures.
Transport Secretary Grant Shapps said the UK’s rail network could not cope with the extreme heat, adding that it would take “many years” before upgrades would mean services could handle the hotter climate.