The temperature has hit 40.2C (104.3F) at Heathrow Airport - a new all-time UK record.
Earlier today, 39.1C (102.3F) was recorded in Charlwood, Surrey - breaking the previous record of 38.7C (101.7F) set in 2019 at Cambridge Botanic Garden.
Temperatures as high as 41C (105.8F) are forecast today, with much of England still under the first ever red weather warning.
The night-time record was broken overnight, as Kenley in Surrey hit 25.8C (78.4F), while Kew Gardens and Heathrow surpassed the previous temperature record by noon on Tuesday, recording temperatures of 38.8C (101.8F).
The sweltering heat has already caused significant travel disruption, with Luton Airport forced to suspend flights on Monday because of a defect in the runway.
On Tuesday, police said a section of a dual carriageway in Cambridgeshire was left looking like a "skatepark" after it warped in the heat.
Trains cancelled and routes closed due to hot weather
Rail users have been warned of delays and cancellations as the heat takes its toll on the country's infrastructure.
There will be no Thameslink or Great Northern trains running in any location north of London, from London Blackfriars via St Pancras, or from London King's Cross or London Moorgate.
Merseyrail said the number of trains running and journey times will be "seriously affected" and some routes closed completely.
LNER will run no trains from south of York and south of Leeds to London King's Cross.
Southern, Southeastern, South Western Railway and Great Western Railway are among the dozens of train companies running significantly reduced services across the country.
Transport for London (TfL) said London's rail network would also be running a reduced service due to safety restrictions put into place to deal with the heat.
'A pretty unprecedented day'
Rachel Ayers, a Met Office forecaster, says Tuesday will be "a pretty unprecedented day", with temperatures possibly reaching highs of 41C (105.8F) in spots in England.
Scotland and Wales could also see their hottest days on record.
Heatwaves like the one we are experiencing this week will "repeat themselves and get more severe going forward in time", climate change expert Sir David King told Sky News.
Bob Ward, policy director at the Grantham Research Institute on Climate Change, told Sky News that the UK "has got to stop thinking of itself as a cold country".
"In the summer months, we are now a hot country," he said.
"There is no excuse for the government's lack of preparedness for this kind of extreme heat event."
(c) Sky News 2022: UK heatwave: New UK record as temperature hits 40.2C - with 41C expected this afternoon
Event Bookings Go Live For Worthing Festival ‘24
Man Charged After Throwing Brick Through Brighton Restaurant Window
Peacehaven Man Disqualified Over High-Speed A27 Pursuit Near Brighton
Appeal After PCSO Assaulted In Uckfield
Man Charged With Rape Of Teenage Girl In Newhaven
Overnight Closures For A22 Forest Row Road Improvements
Brighton Defence Manufacturer's Controversial Planning Application Likely To Be Heard
Over £2.4m Approved For Worthing School Support Centre
Section Of M25 To Close Again Tonight - As Drivers Warned Not To Get Complacent
Plans For 800 New Horsham Homes Refused