The Met Office has issued heatwave alerts across the south and east of England, with temperatures expected to exceed 30C (86F) in some areas.
A level three heatwave alert is in place for the east of England, where high daytime temperatures and warm, humid nights are forecast until Friday.
London and south-east England are on a level two alert, meaning heatwave conditions are also likely this week.
Public Health England urged people to be aware of the health risks of heat.
To stay safe, people should drink plenty of water, wear loose-fitting cotton clothing, and stay out of the sun between 11:00 and 15:00 BST, it said.
Temperatures of 30C have already been recorded in Cambridge, Writtle in Essex, and Heathrow on Tuesday.
Although there is no official definition of a “heatwave” in the UK, the term can be used to describe an extended period of hot weather relative to the expected conditions in an area at that time of year.
This is different to the Met Office’s four heatwave alert levels, which trigger action from health professionals and emergency planners.
The criteria vary by region – for example, the daytime temperature threshold in north-east England is 28C, but in London it is 32C. The average threshold temperature is 30C by day and 15C overnight for at least two consecutive days.
Parts of the UK are currently at level three of four – four being a “national emergency” red alert, which is issued when a prolonged hot spell becomes severe. (bbcnews)