A red warning for extreme heat was first issued in England on Monday and Tuesday, when temperatures could reach 40ºC for the first time.
The Met Office warns of “an exceptional period of heat […] leading to widespread impacts on people and infrastructure “.
“Adverse health effects to the entire population” are expected, “not limited to those most vulnerable to extreme heat”.
Live weather updates from the UK: temperatures will rise by 10ºC
Elsewhere, the existing amber notice has been extended to the rest of England, Wales and parts of Scotland as of Monday.
The Met Office has said the red warning means “adverse health effects” can be experienced and will not be limited to “those most vulnerable to extreme heat”.
They added that the weather could also cause “serious illness or danger to life.”
Find out the five-day forecast of where you live
The Met Office added that “substantial changes will be required in work practices and daily routines.”
The National Weather Service also warns that there is a “high risk of failure of heat-sensitive systems and equipment, which can lead to a localized loss of energy and essential services such as water or mobile phone services.”
There will also be “significantly more people visiting coastal areas, lakes and rivers, which will increase the risk of water security incidents.”
Citizens have also been warned that there are expected delays on the roads, along with delays and cancellations of rail and air travel.