A rare red weather warning for extreme heat and a red heat health alert have been issued.
The red weather warning issued by the Met Office covers a large part of the region, including Bedfordshire, Buckinghamshire, Essex and Northamptonshire, from 09:00 BST on Wednesday until 21:00 on Thursday.
Meanwhile, the red heat health alert issued by the UK Health Security Agency (UKHSA) will cover the whole of the region.
Met Office deputy chief forecaster Mark Sidaway said its red warnings were reserved for the “most severe events”, and temperatures were expected to reach nearly 40C.