A Met Office statement says there is a high probability of heatwave conditions between noon today (Tuesday) and 9am on Friday in parts of England.
This has triggered a Level 2 Heatwave Alert for West Sussex.
The statement says: “There is now high confidence that temperatures are increasing today (Tuesday August 23) with the highest temperatures across eastern parts of England.
“High daytime temperatures along with warm overnight temperatures mean that trigger criteria for Eastern England are very likely to be met, with moderate confidence that criteria will also be met in South-East England and London.
“Warmest conditions are likely to be across central/west East England as well as London and northern parts of South-East England on Tuesday.
“On Wednesday and Thursday (August 24/25) the warmest conditions are expected across the east of East England as well as London and central/east South-East England. The highest temperatures are likely to be more isolated on Thursday as more cloud is expected.”
The statement adds: “The very young, the elderly and the seriously ill are the groups who are particularly at risk of health problems when the weather is very hot. In particular, very hot weather can make heart and breathing problems worse.”
Anyone travelling to the east of England is warned that there is a Level 3 alert in place.
At Level 3 people are advised to: stay out of the sun. Keep your home as cool as possible – shading windows and shutting them during the day may help. Open them when it is cooler at night. Keep drinking fluids. If there’s anybody you know, for example an older person living on their own, who might be at special risk, make sure they know what to do.