Highs of up to 31C (87.8F) are expected as temperatures peak across East Anglia and south east England on Friday and Saturday.
But it is not the hot sunshine of the day that people need to be wary of, but the sweltering night time temperatures which pose the most threat.
Head of health forecasting at the Met Office Wayne Elliott said: "While there is the possibility of daytime temperatures reaching trigger thresholds, it is the night time values which are of real concern. High humidity and the lack of any breeze could make matters worse for people with underlying health problems."
The next few days will see varied weather across the UK, with some places having unsettled conditions and temperatures only reaching the low 20s.
Chief forecaster at the Met Office Frank Saunders added: "There is a 60% chance of some places in East Anglia and south east England reaching 32C but it is important to note that it won't be hot everywhere. Cloud and some rain are expected for a time over northern regions of the UK."
The NHS advises to keep cool by wearing loose cotton clothes, spraying or splashing faces and the backs of necks with cold water several times a day and
staying in the coolest rooms of the home as much as possible. 
And we will be absorbing in the hottest part of the day 
Windows should be kept closed when the room is cooler than it is outside.
The elderly and babies and young children are the most at risk during hot weather spells, so should be regularly checked to make sure they are not suffering.
The highest temperature reached so far this year was 30.9C (87.6F) in Gravesend, Kent, on June 27.
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