The Met Office has issued an amber Extreme Heat warning for parts of southern and eastern England and south Wales.
The warning is valid for Monday and Tuesday and warns of temperatures peaking at 35C (95F).
The nights will also be hot, remaining above 20C in some locations.
It brings the risk of impacts to health, especially for vulnerable people, and the danger of sunburn and heat exhaustion.
Some disruption and travel delays are also likely.
This Met Office warning is separate from the amber and yellow Heat Health Alerts that had already been issued by the UK Health Security Agency (UKHSA).
Those alerts, valid for regions of southern and eastern England until Tuesday, warn of significant impacts to health and social care services – with a likely increase in deaths among elderly and vulnerable people.
In contrast, this Met Office warning suggests that the heat will have impacts on the general population.
This is a developing story and will be updated.