The Met Office took a provisional temperature reading of 40.2C at London Heathrow.
But later in the day, a new provisional UK record temperature was recorded as 40.3C at Coningsby, in Lincolnshire, the Met Office said.
The threshold was hit at 12.50pm as much of the UK sweltered in a heatwave, with parts of England and Wales under a red warning for extreme heat.
This is said to pose a danger to life, pile pressure on the NHS and cause disruption across transport networks.
The new high for daytime temperatures comes after the UK experienced its warmest night on record on Monday, with temperatures remaining in the mid-20s.
Weather warnings are in place across the UK as temperatures reach extreme levels.
Ben Birchall
The Met Office warned temperatures were still climbing early on Tuesday afternoon.
The UK's national weather service tweeted: “For the first time ever, 40 Celsius has provisionally been exceeded in the UK.
“London Heathrow reported a temperature of 40.2C at 12.50pm today.
“Temperatures are still climbing in many places, so remember to stay #WeatherAware.”
As soaring temperatures continued into the evening, train services running out of London Kings Cross, London St Pancras International or London Euston were halted, while only a very limited service running out of London Marylebone.
40.2C has been provisionally recorded at Heathrow Airport.
Yui Mok
Planned surgeries are being scaled back due to the heat in operating theatres.
Peter Byrne
Interim deputy chief executive of NHS Providers, Miriam Deakin, said the heatwave was impacting urgent and emergency care and some planned care.
She said: “Across the country we are seeing hospitals having to scale back the number of planned surgeries as operating theatres are too hot.
“Trusts are having to install industrial cooling units, mounting fans, and trying to cool down IT server rooms.”
She said staff have swapped formal uniforms for cooler scrubs, while in some places those who are not frontline workers have been given the option to work from home, and hospital kitchens are making ice lollies for staff and patients.
Ms Deakin added: “Trusts are checking in with their vulnerable patients to make sure they have ample amounts of water and plans in place to stay cool.
“Many services have reviewed their clinical activities to assess whether appointments or group activities should be cancelled or moved virtually, particularly in community and mental health services.”
Discharge teams were considering the heat to make sure patients, carers, community hospitals and care homes have water supplies before being discharged, and the amount of time patients spend in a car or ambulance is reduced, she said.