People are being urged to "stay out of the sun" for most of this week, as a heat health watch alert 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.
The hottest day of the year has been recorded at Santon Downham in Suffolk, which reached 33.3 C on Monday (92F).
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.
In the affected areas there is a 90% probability of heatwave conditions, the Met Office said.
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 30C 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," a 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 new normal?
By David Shukman, BBC science editor
Heatwaves are nothing new for the UK.
I was struggling through my A-level exams during the blazing weather of 1976 when for 15 days running temperatures were above 32C somewhere in the UK.
This year has not matched that. So far, at least.
But since those overheated schooldays the planet has changed. The average temperature has risen.
And scientists are convinced that we are starting to feel the effects of humanity's greenhouse gases warming the atmosphere. They point to the multiple heatwaves under way in different parts of the globe at the same time.
While parts of the UK are sweltering, there's stunning warmth in Scandinavia even inside the usually chilly Arctic Circle and in Japan dozens of people have reportedly died from record temperatures.
The latest research into the future of the climate does not say that we will get heatwaves every year. The next few summers may well be cool.
But studies do suggest that heatwaves will become more frequent, and that when they happen they will last longer than they do now.
So children currently enjoying the holiday sunshine may well grow up into a world where heatwaves seem almost normal.
How does this compare to the UK's hottest summers?
- In 1976, there were 15 days running where somewhere in the UK was over 32C. That record has not been matched by the 2018 heatwave yet
- In 1969, there were 70 days in one part of Suffolk with less than a millimetre of rain in total. Two places in west London have so far gone 54 days with the same low rainfall
The Met Office said the first half of summer has been the driest since 1961. Several parts of the country have had 54 consecutive dry days, meaning less than 1mm of rain fell.
The longest run with no rain at all has been 48 days at Brooms Barn, near Bury St Edmunds. The Met Office said most of its weather stations in East Anglia have had no rain at all since 21 June.
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 the BBC.
"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."
'The Sussex savannah'
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."
Sussex farmers say crops are "shriveling-up" and dying in the heatwave. pic.twitter.com/jC6XjELIgd— BBC Sussex (@BBCSussex) July 23, 2018
David Exwood, who runs Westons farm in West Sussex, said his yields are down by 25%.
"Crops are suffering, the fire risk at the moment is exceptional," he told the BBC.
"It's the Sussex savannah at the moment. I mean the cracks in the ground are extraordinary - they go down over a metre and so it's going to take a lot of rain to turn this around."
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 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."