Health chiefs issued a heat alert on Tuesday afternoon as temperatures were set to soar to up to 34°C in coming days.
The UK Health Security Agency acted after the Met Office warned of high temperatures later this week.
The alert, which comes into place at midnight on Thursday until midnight on Saturday, covers London, the South East, the East Midlands, East of England and South West regions.
Dr Agostinho Sousa, Head of Extreme Events and Health Protection at UKHSA, said: “High temperatures are expected in some parts of the Midlands and the south on Friday. We want everyone to enjoy the hot weather safely when it arrives and be aware of good health advice for coping with warmer conditions.
“During periods of hot weather, it is especially important to keep checking on those who are most vulnerable, such as older people and those with heart or lung conditions. Make sure to look out for signs of heat exhaustion and follow our simple health advice to beat the heat.’’
Dan Rudman, Deputy Chief Meteorologist at the Met Office, added: “Temperatures will continue to rise as we go through the week, becoming well above-average by Friday, when many parts of the southern half of the UK are likely to exceed 30°C or even reach 34°C in some places.
“This is the first spell of hot weather this year and it is unusual for temperature to exceed these values in June. Many areas will also see some warm nights with minimum temperatures expected to be in the high teens or even low 20s for some overnight.’’
The health chiefs stressed that ways to keep safe in the sweltering heat include:
• try to keep out of the sun between 11am to 3pm, when the UV rays are strongest
• drink plenty of fluids and avoid excess alcohol
• look out for those who may struggle to keep themselves cool and hydrated – older people, those with underlying conditions and those who live alone are particularly at risk
• stay cool indoors by closing curtains on rooms that face the sun – and remember that it may be cooler outdoors than indoors
• never leave anyone in a closed, parked vehicle, especially infants, young children or animals
• walk in the shade, apply sunscreen and wear a wide-brimmed hat, if you have to go out in the heat
• avoid physical exertion in the hottest parts of the day
• make sure you take water with you if you are travelling
• take care and make sure to follow local safety advice if you are going into the water to cool down
This is the first Level 2 heat-health alert of the year.