A heatwave warning has been issued by the Met Office for condition from Friday to Sunday.
The level two caution includes London and Essex where temperatures could rise over 30 degrees during the weekend and remaining above 15 degrees at night.
Humidity is also expected to be high.
Information on the NHS website suggests elderly and very young people are affected most by high temperatures and are advised to avoid going outside between 11am and 3pm.
People are also advised to drink water or fruit juice throughout the day and avoid tea, coffee and alcohol.