The Met Office has warned that people’s lives could be at risk as it indicated it is likely that a new UK record temperature could be set early next week.
Saturday is set to start with overcast skies and scattered showers in northern areas, before turning dry across the nation as the extreme heat moves in.
Daytime temperatures on Saturday are predicted to be around 27C in London, 26C in Cardiff, 23C in Belfast and 21C in Edinburgh.
On Sunday, the mercury will rise to 30C in the capital, 27C in Cardiff, 24C in Belfast and 23C in Edinburgh.
Temperatures are forecast to increase by several more degrees across the nation on Tuesday – up to the mid-thirties for much of England and Wales.
There is a 50% chance of temperatures reaching 40C somewhere in the UK, likely along the A1 corridor, with the Met Office issuing its first ever red warning for extreme heat.
The UK Health Security Agency has increased its heat health warning from level three to level four – a “national emergency”.
Level four is reached “when a heatwave is so severe and/or prolonged that its effects extend outside the health and social care system… At this level, illness and death may occur among the fit and healthy, and not just in high-risk groups,” it said.
A Met Office red warning, for Monday and Tuesday, covers an area from London up to Manchester, and up to the Vale of York.
Met Office spokesman Grahame Madge said: “If people have vulnerable relatives or neighbours, now is the time to make sure they’re putting suitable measures in place to be able to cope with the heat because if the forecast is as we think it will be in the red warning area, then people’s lives are at risk.
“This is a very serious situation.”
A No 10 spokesman said railway speed restrictions may be needed on “some parts of the network next week to manage the hot weather and to avoid any potential damage”.
Jake Kelly, of Network Rail, warned that journeys will take “significantly longer and delays are likely as speed restrictions are introduced to keep passengers and railway staff safe”.
Train operators have warned passengers to avoid travelling on Monday and Tuesday unless their journey is “absolutely necessary”.
Some southern schools are also considering closing on these days – and the NEU teaching union issued a statement saying it would support headteachers taking this decision.
Downing Street said that Cobra met on Thursday amid the heatwave, and discussions with sectors including the NHS will “continue to work closely with all of those sectors over today, through the weekend and into early next week”.
Meanwhile, motorists have been advised to try to make their journeys outside of the hottest periods of the day, particularly if they have older cars.
North Wales Police said a paddleboarder died after getting into difficulty in the water off Conwy Morfa beach on Thursday evening.
Officers were called at 10.14pm and attended alongside the coastguard, RNLI and ambulance service, with two casualties taken to hospital in Bangor, where one of them, a 24-year-old woman, died.