The Met Office issued an amber "heat health watch warning" for parts of England this week, urging people to either stay out of the sun or avoid being in the sun when it is at its strongest between 11am and 3pm.
The amber, or level three, warning is issued when temperatures are predicted to hit 30 degrees during the day, and 15 degrees at night, for at least two consecutive days, the Met Office spokeswoman said.
There is a 90% possibility of heatwave conditions between 9am today and 9am Friday in parts of England, mainly in the south and east, she added.
The heat health watch warning is designed to make local services aware that these conditions are being met, and for them to take action.
"We advise the public to take care in the sun, especially when temperatures are potentially reaching 30 degrees or more throughout this week - either stay out of the sun or be sensible and don't go out in the strongest sunshine hours (11am to 3pm)", the Met Office spokeswoman said.
Members of the public were also urged to take the usual precautions in the sun, including covering up, wearing sun screen and drinking plenty of water.