People are being urged to “stay out of the sun” for most of this week, as a heat health watch warning is issued by the Met Office.
The level 3 or amber alert is in place until 09:00 BST Friday for much of the east and south-east of England.
In these areas there is a 90% probability of heatwave conditions, the Met Office said.
The National Farmers’ Union has warned of crops “parched to the bone” and livestock farmers using winter rations.
The level 3 warning is issued when temperatures are predicted to hit 30C during the day, and 15C at night, for at least two consecutive days.
It is the third time the level 3 alert has been reached in England this year.
The alert is different to the severe weather warnings issued throughout the year for snow or rain.
Operating in conjunction with Public Health England, the heat health watch service helps keep health professionals and people working in social care prepared to keep people safe.
“We advise the public to take care in the sun, especially when temperatures are potentially reaching 30 degrees or more throughout this week – either stay out of the sun or be sensible and don’t go out in the strongest sunshine hours – 11am to 3pm”, the Met Office spokeswoman said.
The public were urged to take the usual precautions in the sun, including covering up, wearing sun screen, keeping your house cool and drinking plenty of water.
The warning follows weeks of dry, hot weather, which has already caused a hosepipe ban, closed canals and revealed ancient hidden landscapes.
Meanwhile farmers across England and Wales will be hoping for the heatwave to end.
Guy Smith, deputy president of the National Farmers’ Union said crops were “being parched to the bone”.
“Spring crops that farmers sowed in April barely knowing what rain is,” he told BBC news.
‘The signs are ominous’
“As you travel West… a little bit more rain but even here the grass has stopped growing and that’s a problem for livestock farmers because they need to have that grass growth to sustain their sheep and cows through to the winter.
“We’re hearing that livestock farmers are now having to feed winter rations to their stock and that’s going to cause problems later on.”
Mr Smith said vegetable farmers also face problems as their reservoirs – used for irrigation – begin to run dry.
“If this weather continues… we will see impacts on vegetable production. The signs are ominous.”
Meanwhile, the Woodland Trust has warned wild berries are ripening early, which could lead to them being smaller or dropping from trees and shrubs.
“We’re already anticipating signs of autumn,” said Dr Kate Lewthwaite from the Woodland Trust.
“Although we’ve only had a small number of berry records so far, the heatwave will only encourage more fruit to ripen, and leaves on trees may also start to change colour.”