Friday could be the hottest ever July day, before thunderstorms provide slight relief from the scorching heatwave.
After weeks without rain, torrential downpours are forecast to hit parts of the UK, with as much as 30mm (1.2in) expected to fall in just an hour.
A sizzling 36C (96.8F) is forecast for the south east, and a thunderstorm warning is in place for eastern and northern parts of England from Friday afternoon.
With a chance of hail and strong winds alongside the torrential downpours, the temperature could drop back to the high 20s, meteorologist Alex Burkill said.
Cavendish, in Suffolk, was the hotspot on Tuesday with the temperature reaching 31.4C (88.5F), 23C (42.1F) higher than the lowest temperature recorded, at Drumnadrochit, near Loch Ness in the Highlands, at 8C (46.4F).
Wednesday is due to be warmer, with temperatures of 33C (91.4F) expected in East Anglia and the south-east.
Nurses have warned that some hospital wards have reached more than 30C (86F), leading to patients and relatives passing out or vomiting.
The Met Office has said England could see the July record high of 36.7C (98.1F), reached at Heathrow in 2015, broken on Friday.
The all-time UK high is 38.5C (101.3F) set in August 2003.
And pet owners have been urged to avoid exercising their dogs and other animals in the hottest parts of the day due to the risk of burnt paws on scorching pavements, as well as the chance of heatstroke.
And as the harvest begins early in many places following the driest first half of summer on record, farming leaders warned crops are being hit.
They added that reservoirs for watering vegetables were running dry and livestock owners were having to use winter feed for their animals as summer grass had withered away.
Meanwhile, the heatwave has also seen a day’s play lost in the India warm-up match against Essex. The fixture will now be a three-day affair following concerns about the prolonged hot weather.
The heatwave baking Britain comes as extreme hot weather grips northern Europe.
The UK has seen the driest half of summer on record, with just 47mm (1.85 inches) of rain between June 1 and July 16.
The Met Office said several places have had 54 consecutive dry days, starting on May 30, including a few which have had less than 1mm (0.04in) of rain in the entire 54-day period – the longest spell since 1969, when 70 days passed with no significant rainfall.
The longest run of days with no rain at all this summer so far is 48 days at Brooms Barn, near Bury St Edmunds, since June 5.
Conditions this week have been nearer normal for the time of year in Northern Ireland and western Scotland, which are seeing lower temperatures and rainy spells.