A yellow heat health alert is in place from midday for most of England, with temperatures expected to peak in the mid-30s tomorrow.
Parts of the country are facing a fourth heatwave of the summer, where highs pass a threshold on three consecutive days.
Check the weather forecast where you are
Sky News meteorologist Christopher England said the high pressure that brought the warmth of the last few days via the "heat dome" effect is moving east, as low pressure moves in towards the west.
This will bring even warmer air up from the near continent, making it hotter for most over the next few days.
"Southern Britain can expect temperatures widely into the low 30s then, perhaps exceeding 35C (95F) in places," Mr England said.
"There's around a 10% chance Wales may exceed its august peak temperature of 35.2C recorded at Hawarden on 2 August 1990."
He also predicts "some very muggy nights" in the South, with temperatures quite widely holding above 20C (68F) in towns and cities, known as "tropical nights".
A yellow health alert is in place from 12pm on Monday through to Wednesday evening for most of England - covering all regions except for the North West and North East.
The warning issued by the UK Health Security Agency means it expects heat-related issues such as an increase in deaths of over-65s, a higher demand on health services and an increased risk of overheating for vulnerable people.
The Met Office's criteria for a heatwave are met when temperatures are above a certain level for three consecutive days. This threshold varies from 25C to 28C (77F to 82F) depending on location.
Meteorologist Tom Morgan said there would be a "North-South split in the weather" today.
He said it would be "quite cloudy across Scotland, Northern Ireland and parts of northern England, the rain tending to come and go, but most persistent in western Scotland".
The remnants of ex-tropical storm Dexter has headed towards the UK from the Atlantic.
This could bring the potential of rain and thunderstorms tonight and into tomorrow.
(c) Sky News 2025: UK weather: Hottest day of heatwave revealed - with 10% chance temperatures will hit 35C