A national emergency has been declared after a red extreme heat warning was issued for the first time, as temperatures could hit 40C (104F)
The Met Office's highest warning covers an area including London, Manchester and York on Monday and Tuesday.
It means there is a risk to life and daily routines will need to change.
Speed restrictions are likely on railway lines, some schools will close early and some hospital appointments will be cancelled.
On the roads, gritters are planning to spread sand to reduce melting, and the RAC has warned more drivers will need help as cars overheat.
Meanwhile in sport, five horse race meetings have been cancelled on Monday and Tuesday because of the forecast temperatures.
On top of the Met Office warning, the UK Health Security Agency issued its highest level four heat alert to health and care bodies - warning illness and death could occur "among the fit and healthy".
It is the first time a red heat warning has been issued for parts of the UK, although the extreme heat warning system was only introduced in 2021.
Downing Street said the alert was being treated as a national emergency, with officials meeting on Friday and over the weekend to discuss the response.
The weather at the start of next week is forecast to be warm across the UK but temperatures will be more like 30C in Scotland and Northern Ireland, rather than the high 30s predicted in parts of England and Wales.
The heatwave - originating in north Africa - is spreading across Europe and has fuelled wildfires in Portugal, France and Spain.
Met Office CEO Penny Endersby said the extreme heat forecast was "absolutely unprecedented" and she urged people to take the warning as seriously as a red or amber warning for snow or wind.
There have been concerns about pressure on the NHS during the extreme weather. Downing Street said contingency plans were in place and the prime minister was being updated.
