The UKHSA heat-health alert, the first of its kind this year, is active from 9pm tomorrow until 8am on Sunday and covers the East of England, South East, East Midlands and London.
Sunday will see most of the UK facing medium or high alerts, while very high warnings will remain in place for London and the South East of England, the East, the West Midlands and the East Midlands.
The Met Office said high pressure sitting to the South East would 'draw up some chilly air' across southern areas this weekend with conditions also expected to be cloudy.
The National Audit Office (NAO) has urged ministers to put 'greater focus' on ensuring more devices in rural locations and regions outside the South East.