For the first time in its history, the British Weather Service has issued an extreme heat warning.
"Exceptional, possibly record-breaking, temperatures are possible on Monday and then again on Tuesday," the Met Office said on Friday.
Nights would be exceptionally warm for Britain, especially in towns.
"This is likely to result in widespread human and infrastructure impacts," the agency said.
Schools announced earlier closing times in response.
Instead of school uniforms, sportswear can be worn in some schools and sporting events are postponed.
The warning applies to a large area between London, Manchester and the York Valley.
The British heat record is 38.7 degrees Celsius - measured on July 25, 2019 in the Botanic Gardens of the University of Cambridge.
Meteorologists expect that this value will be exceeded.
Met Office spokesman Grahame Madge even fears temperatures of 40 degrees.
"When it hits 40 degrees, that's a neuralgic threshold that shows climate change is upon us now," Madge said.
The Met Office had previously warned of health risks from heat - level 3 on the warning scale.
Now the authority has announced level 4.
According to the statement, this applies “when a heat wave is so severe and/or long-lasting that its impacts go beyond the health and social systems.
At this level, disease and death can occur among fit and healthy people, and not just in high-risk groups.”