Temperatures are set to rise by another 10C in much of England over the coming days, increasing the strain on hospital and train services.
On average, the mercury will remain in the mid-20s for the last working day of the week, before rising to around 35C (95F) in southern, central and eastern areas of England by Tuesday.
Hospital and train services are already under pressure from the heat, with the NHS facing a “surge” in demand from heat-related admissions, and Avanti West Coast has warned their services may be cancelled at the last minute.
The Met Office has issued an “amber” extreme heat warning for much of England and Wales from Sunday until Tuesday.
Tuesday is likely to be the hottest day, with the potential to top the UK record temperature of 38.7C (101.7F) set in Cambridge in 2019, forecasters said.
The warning says the weather could cause health problems across the population, not just among people vulnerable to extreme heat, leading to potentially serious illness or danger to life.
Friday will begin with outbreaks of showery rain moving south-east across northern and central areas of the UK.
Meanwhile, people in the South and South West can expect warm sunshine.
London is forecast to experience 26C on Friday, while 23C is predicted for Cardiff, 20C in Belfast and 19C in Edinburgh.
Saturday is also set to start with overcast skies and scattered showers in northern areas, before turning dry across the nation as the extreme heat moves in.
During the peak of the heatwave on Tuesday, 36C is predicted in the capital, 28C in Cardiff, 26C in Belfast and 25C in Edinburgh.
Cabinet Office minister Kit Malthouse has said the Government is preparing for a “surge” in demand on the NHS and other services due to the expected heatwave.
After chairing a meeting of the Cobra civil contingencies committee in Whitehall, Mr Malthouse urged the public to look out for people who were particularly vulnerable in the heat.
Avanti West Coast has warned passengers who wish to use services between London Euston and Scotland to plan ahead due to the impact of the heat on rail lines.
The company said that weekend services may be cancelled at short notice while journeys on Monday and Tuesday will be subject to amended timetables.