An amber heat-health alert has been issued for the South East.
The alert covers regions including Sussex and Surrey and will remain in place until Tuesday, June 23.
Temperatures are expected to rise above 30C on Friday, with an amber alert warning that conditions could have a wider impact on the population and place additional pressure on health services.
Professor Andrew Rhodes, joint chief medical officer at NHS Surrey and Sussex, said: "Hot weather can pose serious health risks, especially for older people, babies and young children and those living with long-term health conditions.
"During periods of prolonged high temperatures, there can be an increase in heat-related illness and additional pressure on health and care services."
He encouraged people to stay hydrated and take precautions in the heat.
He said: "We encourage everyone to take sensible precautions to stay well in the heat and to look out for people who may be more vulnerable.
"Simple measures such as drinking plenty of fluids, keeping cool indoors and avoiding the midday sun can make a real difference."
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The public is advised to keep homes cool by closing windows and curtains in sun-facing rooms.
If heading outside, people should wear light clothing, a hat and sunglasses, use sunscreen and stay in the shade during the hottest hours between 11am and 3pm.
Physical activity should be planned for cooler parts of the day, such as early morning or evening.
People are urged to look out for signs of heat exhaustion and heatstroke in themselves and others.
The NHS is reminding the public that 999 and A&E services are for life-threatening emergencies only and should be used if someone is seriously ill or injured.
Other available services include pharmacies for advice and over-the-counter medicine, GP practices for non-urgent medical needs and NHS 111 online or by phone for guidance on the most appropriate care.
Those needing urgent care that is not an emergency can also visit walk-in centres and urgent treatment centres.
For more information about staying safe during hot weather, visit the UK Health Security Agency and NHS websites.
