Britain has officially experienced its second hottest day ever with temperatures reaching a scorching 38.1C in Cambridge.
A blazing 37.7C was also recorded at London Kew Gardens and in Writtle, Essex, on Thursday afternoon.
It comes just hours after the heatwave broke the all time hottest day for July, with the mercury passing 36.9C.
The Met Office confirmed the record breaking temperature was reached at Heathrow Airport, beating the previous record of 36.7C - also set at Heathrow in 2015.
People are being urged to take precautions against the heat, including staying hydrated, staying inside at the hottest time of the day, avoiding exercise and wearing loose, light clothing.
Medical experts are warning that few lessons have been learned from last year's heatwave, and few hospitals are prepared for the impact of intense heat.
Nick Scriven, president of the Society for Acute Medicine, said NHS staff were "struggling" and "overheated and exhausted staff" were at greater risk of making errors.
"Last year, hospitals hired in large fans and coolers for a week or so but have got nothing long-term in place - they are purely reactive not proactive," he said, adding there was often little in place for staff to get fluids on wards.
A yellow weather warning is in force for most of England, except the south-west, and parts of Scotland from 3pm on Thursday until 4am on Friday.
"It will get into the 30s across the country and reach the mid-30s in the south-east," Met Office meteorologist Greg Dewhurst said.
Commuters have been warned not to travel as the extreme heat could cause tracks to buckle, bringing about speed restrictions, delays and cancellations.
London North Eastern Railway (LNER), which runs inter-city services on the East Coast Main Line, advised customers against travelling on Thursday.
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It said some services were likely to be cancelled or delayed as speed restrictions were imposed between Peterborough and London King's Cross.
Network Rail said extreme weather action teams (EWATs) had been "activated" to keep passengers safe and trains running.
The hot weather has also been interfering with signals for analogue and digital radio signals.
The scorching conditions may also spark thunderstorms this evening which could trigger travel delays, flash flooding, and power cuts.
The dangers of cooling off in lakes, rivers and the sea were highlighted by emergency services after the bodies of three people were pulled from the water on Wednesday after they reportedly got into difficulty swimming.
The Met Office has warned heatwaves are on the increase as a result of climate change.
The scorching temperatures gripping the UK and much of Europe come against a backdrop of global warming of 1C since the Industrial Revolution driven by greenhouse gas emissions.
Both Belgium and the Netherlands broke their all-time maximum temperature records on Wednesday, reaching 39.9C and 39.1C respectively, the Met Office said.
Higher temperatures are making extreme hot spells more likely and more intense, experts warn.