There will be tropical nights in some locations where overnight temperatures are expected to remain above 20C (68F).
This combination of hot days and very warm nights is likely to bring a range of impacts to people and infrastructure.
There is a risk to health, especially for elderly and vulnerable people.
The UK Health Health Security Agency has already issued separate amber and yellow heat health alerts across large swathes of England.
However, even in younger and healthier people there is a risk of health conditions such as sunstroke and heat exhaustion.
Delays and disruption to travel networks and power supplies are possible.
There is also the potential for scattered intense thunderstorms, especially for parts of England and Wales, which could pose an additional hazard.