Eastern and southern England, as well as the Midlands, have had their weekend heat-health alert upgraded from a yellow to a more serious amber warning.
The amber alert, in effect starting at 9:00 BST on Friday, warns that high temperatures could harm people of all ages and have an adverse effect on the health system.
The UK Health Security Agency (UKHSA) has extended the alert until Monday morning at 9:00 BST.
There may be some thunderstorms and temperatures expected to reach 30C.
For the north of England and London, a less serious yellow alert is in effect with the recommendation that people check on vulnerable family members and friends.
The UKHSA initially issued a heat alert on Wednesday but later upgraded it, stating that the affected areas would experience a rapid rise in temperature with high overnight temperatures.
On Saturday from 14:00 BST until 21:00, the entire country of Wales and a sizable portion of southern England are under a Met Office yellow thunder warning.
Due to the strongest showers, there is a chance of travel delays, power outages, and some localized flooding.
