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.
I was up till half 4 and it was just thunderstorms for over an hour, still going when I fell asleep. I don’t think I’ve seen so much lightning.