The heatwave in the UK is expected to hit a peak on Monday and Tuesday, with a 50 percent chance of temperatures hitting 40C, according to the Met Office.
The Met Office has issued a rare red warning because of the heatwave from midnight on Sunday.
"Substantial changes in working practices and daily routines will be required," the Met Office said.
The warning covers an area from London up to Manchester in the west and then over to the Vale of York.
In turn, the UK Health Security Agency (UKHSA) increased its heat health warning to level four, which means "a heatwave is so severe and/or prolonged that its effects extend outside the health and social care system".
The national weather service also warned that delays on roads, along with delays and cancellations to rail and air travel, could be expected.
Major work was being done by the UK government to ensure that the most vulnerable would be protected during the heatwave, Downing Street said.