Thursday could be the hottest day recorded in the UK, with temperatures likely to reach 39C (102.2F), the Met Office says.
The current record is 38.5C (101.3F), which was seen in Faversham in August 2003 but there is a 70% chance temperatures will soar above this on Thursday, according to the Met Office.
Met Office meteorologist Emma Smith said: "It's normally 13 or 14 degrees at this time of year, so that's 10 degrees above average."
There is also a possibility the highest overnight temperature of 23.3C (73.94F) will be passed on Thursday night.
By early Wednesday, temperatures were already well above average, with a number of areas across the UK hitting 23C (73.4F) to 24C (75.2F) by 5am.
Temperatures soared into the early 30s by the end of the day.
The extreme temperatures have prompted warnings from health professionals to stay out of the sun, drink plenty of water and to check on older people who live alone.
Animal welfare charity Dogs Trust has also warned owners not to leave their pets in hot cars as it can prove fatal.
Meanwhile, employers are being advised to help their workers keep cool by allowing flexible working to avoid the rush hour, home-working, relaxing dress codes and keeping buildings cool.
On Tuesday, a jury deliberating over a murder case were sent home early after complaining about "extreme heat" in the deliberation room.
There is also some travel disruption expected on Thursday, with Southeastern and London North Eastern Railway among those warning of a reduced service.
Both operators blamed speed restrictions imposed by Network Rail due to the heat.
LNER said these restrictions would be in effect between Peterborough and London King's Cross.
Nick King, of Network Rail, said: "Keeping passengers safe and moving are our top priorities during this heatwave.
"That's why we sometimes have to put speed restrictions on to prevent our rails - that can be over 20 degrees hotter than air temperatures - from buckling, which can derail a train and cause huge delays."
Police have also warned about the dangers of swimming in open water after the recovery of three bodies in a Gloucestershire water park and the River Thames.
On Tuesday night the country was hit with a series of thunderstorms and lightning strikes, with a national thunderstorm warning put in place.
Heavy and potentially thundery showers will be developing more widely later on Thursday, but the southwest will turn dry.