The amber heat health alert – issued by the UK Health Security Agency, external – begins across the Midlands, East Anglia, London and South East England at 09:00 and continues until 18:00 on Wednesday.
North East England, North West England, Yorkshire and Humber and south-west England have a yellow alert for the same period.
Yellow alerts warn of possible impacts on health and social services.
Temperatures across Scotland and Northern Ireland could reach 23-26C, maybe 27C in eastern Scotland and the Borders.
Much of England and Wales could see 25-28C, but the low 30s are again likely for central and southern England, and south-east Wales. Anywhere from the south-west Midlands to west London could see 34C.
This would still not make it the warmest day of the year so far, although the year’s top temperature in Wales may be threatened.