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.
The developing heatwave is likely to have widespread impacts on people and infrastructure.
It brings the risk of impacts to health, especially for vulnerable people, and the danger of sunburn and heat exhaustion in the wider population.
People travelling to coasts, lakes and rivers are being warned to stay safe in and around water.
Some travel delays and disruption are also likely.
This is only the sixth amber extreme heat warning that has been issued by the Met Office since they were introduced in 2021 – and the first since August 2022.
A more severe red warning was also issued in the July of that year.
It 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 effects on the general population.