Britain's meteorological agency issued its first-ever extreme weather warning, forecasting that the mercury could climb to 33°C later this week.
The Met Office said the Amber Extreme Heat Warning covers all of Wales, all of southwest England and parts of southern and central England.
The UK has been under heatwave conditions in recent days but temperatures will climb further this week, possibly reaching 33°C in some western parts, the Met Office said.
These "unusually high temperatures" as well as "continuing high nighttime temperatures" can have potential impacts for heath, it noted.
Public Health England (PHE) has meanwhile extended its Heat Health Alert to Thursday. It is currently at Level 3, which requires social and healthcare services to target specific actions for high-risk groups.
Dr Owen Landeg, Scientific and Technical lead at PHE stressed that "it is important to keep checking on those who are most vulnerable such as older people and those with heart and lung conditions."