Temperatures have reached 40C (104F) for the first time on record in the UK.
London Heathrow provisionally recorded 40.2C (107F) at 12.50pm on Tuesday, the Met Office said.
Earlier this afternoon, the national weather agency had already announced the hottest day on record when Charlwood, Surrey, hit 39.1C - warning it could get even hotter.
The threshold comes as much of the UK swelters in a blistering heatwave, with parts of England and Wales under a red warning for extreme heat.
Brits have been warned the weather poses a danger to life, pressure on the NHS and disruption across transport networks.
The abnormal weather is fuelled by climate change, which is making every heatwave more intense, frequent and likely, scientists say.
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.
To keep up to date with the latest heatwave developments, see the Mirror's live blog
The Met Office warned temperatures were still climbing early on Tuesday afternoon.
The national weather agency 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."
Authorities have put Britain, which often struggles to maintain key transport services when hit by unexpected weather such as heavy snow or high winds, on a state of "national emergency" over the unprecedented temperatures.
Transport minister Grant Shapps said it would take many years to fully upgrade Britain's infrastructure to cope with higher temperatures, after at least two airport runways showed signs of damage and some train tracks buckled.
"We've seen a considerable amount of travel disruption," he told the BBC. "Infrastructure, much of which was built from the Victorian times, just wasn't built to withstand this type of temperature."
In recent days, roads have melted and firefighters have been battling wildfires.
The country is set to be hotter than Jamaica, the Maldives and Barbados today – with rail users warned of delays, cancellations and changes to train services.
There have been warnings of pressures on hospitals from the extreme temperatures, and concerns ambulance services would face rising numbers of calls as the heat peaks.
At least 12 people have drowned while swimming in open water amid the hot temperatures this month - despite warnings.
The scorching temperatures, however, will come to an abrupt end later this week as heavy rain is set to fall across much of the UK, according to weather forecasters.
The Met Office has said things will cool down dramatically on Wednesday as the scorching weather is set to disappear, with downpours predicted across the country.
