As temperatures rise to the high 30s, the Met Office warns that the heat will impact health, infrastructure and property.
Red warnings are rare and only issued in exceptional circumstances. The last red warning for extreme heat was issued in July 2022.
This was also when the UK temperature record of 40.3C was set in Coningsby, Lincolnshire.
With forecasts suggesting we could see 40C on Wednesday or Thursday in parts of England, this record may be under threat.
UKHSA red heat health alerts, which also run through Wednesday and Thursday across the Midlands, eastern and southern England, indicate that severe impacts are expected across health and social care services, with an increased risk to life across the whole population.
Heat health alerts are intended to inform the health and social care sector and the responder community.
Amber heat health alerts have also been issued across Northern England for the same times.