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.
“The last time the UK recorded temperatures this high was in July 2022, but on this occasion the heat is expected to be accompanied by high humidity,” he said.
He added there would be consecutive nights where temperatures did not drop below 20C.
Would like to take this opportunity, sitting here in my sweltering house, to note that instead of doing something useful like making installing external shutters permitted development, Kier instead decided to take this many years to block porn for puritan Americans at the behest of Mandleson and Palantir