The UK Health and Security Agency (UKHSA) has upgraded its heat-health alert for South East England.
It said its previous yellow warning for the region was now amber, meaning the whole population could be at risk from hot weather.
The warning will be in place from 9:00 BST on Tuesday to the same time on Thursday.
It comes as temperatures are expected to peak in the mid-30s Celsius on Wednesday.
The UKHSA’s amber heat-alert applies to the East Midlands, East of England and London, alongside Kent, Surrey and Sussex.
It means the NHS may also see increased demand on GPs and ambulances, plus travel disruption is likely.
Other risks identified by the government agency include an increase in water-related incidents, such as people getting into trouble swimming in seas and rivers.
This latest heatwave comes after some of the driest regions of the UK missed a chance of rain over the weekend.
Parts of southern, central and eastern England remain in “prolonged dry weather” -the final stage before drought is declared.
Wisley in Surrey has been one of the driest locations in southeast England with 40 days of no rainfall up to and including 26 July.
Dry ground and low soil moisture levels are likely to become an increasing concern, potentially fuelling issues with water supply, wildfires and agriculture.
Extreme weather events are becoming more common and more intense around the world, fuelled by human activities such as the burning of fossil fuels.
Both Surrey and Sussex have been hit by several wildfires in recent weeks.
Kent Fire and Rescue Service (KFRS) said on Monday it had been called to more than 300 accidental grass fires since the start of May.
Nick Scott, from KFRS, said these types of fires can have “devastating consequences… such as injuring or killing wildlife, destroying property and livelihoods, or even putting someone’s life at risk.”
