A national emergency has been declared after a red extreme heat warning was issued for the first time, as temperatures could hit 40C.
The Met Office’s highest warning, meaning there is a risk to life, covers an area including London, Manchester and York on Monday and Tuesday.
The UK Health Security Agency has also issued its highest level four heat alert to health and care bodies.
It warned illness and death could occur “among the fit and healthy”.
Downing Street said the alert was being treated as a national emergency, with officials continuing to meet on Friday and over the weekend to discuss the response.
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.
As well as the red warning an amber warning will be in place across most of England on Sunday and all of England, Wales and the south of Scotland on Monday and Tuesday.
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 Met Office said the hot weather was caused by high pressure over the UK and hot air from southern Europe.
Night-time temperatures in some areas will not drop below 25C, he said, adding that things were likely to start cooling off from Wednesday.
In England, there were 2,500 excess deaths in the summer of 2020 due to hot weather, and the Red Cross predicts that heat-related deaths in the UK could treble in 30 years.