The Met Office has said that temperatures could hit 30C earlier in the year than ever recorded on Thursday.
A corridor of heat stretching between London and Bristol is likely to see temperatures 10 degrees above average for the time of year, with the record-breaking heat raising alarms about heatwaves becoming more frequent.
Meteorologist Jonathan Vautrey said Thursday may see a likely top temperature of around 29C in areas between London and Bristol, but temperatures could rise to 30C in some spots.
Mr Vautrey added: “That corridor between the two cities could well see some of the top temperatures over the course of this hot spell.”
In London itself, temperatures are expected to reach 28C on Thursday, with similarly hot temperatures predicted for Wednesday too.
While it is not particularly unusual to see warm and sunny periods in April, where the temperature often reach the mid-20s, it is uncommon for the mercury to climb into the high 30s.
Temperatures for the end of April usually sit at around 16C in London, while the highest recorded April temperature was in 1949, when Camden Square, London, recorded 29.4C.
For May, the highest temperature recorded was 32.8C on May 22 1922 at the same location.
The climbing temperatures mean that the UK could see the hottest day of the year so far recorded day after day this week, Met Office meteorologist Craig Snell said.
"We're certainly on track to see the warmest spell of the year so far," he said, adding that temperatures should fall overnight to the low teens, so "we shouldn't suffer too much with warm nights".There is a possibility the UK could see heatwave conditions this week, if temperatures reached 25C on Tuesday, Wednesday and Thursday, the Met Office said.
According to the Met Office, the definition of a heatwave is three consecutive days of temperatures exceeding the “heatwave threshold”, which varies across the country.
The threshold is 25C for most of the UK, with slightly higher numbers for the South and East, and rising to 28C in London.
Temperatures are expected to start to dip across much of the country on Friday, with the mercury returning to the mid-teens by the end of the Bank Holiday weekend.
Mike Childs, Friends Of The Earth head of policy warned that heatwaves will become “far more frequent and more intense as climate change takes hold”.
“The UK also needs to go further and faster to cut its emissions,” he said.
“A new climate action plan is due in October, and ministers must seize the enormous opportunities this will bring.
“As well as getting UK climate targets back on track, it will also cut bills, create new jobs, insulate our heat-leaking homes – and put the UK at the forefront of helping to fix our broken planet.”
