People are being warned of heat risks to their health amid temperatures reaching above 30C.
The UK heatwave is expected to hit a peak on Monday as temperatures build towards 35C in parts of the UK.
A rare amber warning for much of England and Wales has been issued from midnight on Sunday.
That means there could be a danger to life or potential serious illness from the scorching temperatures.
Extreme heat is hitting Europe with Seville reaching 46C while France, Germany and Italy are expected to experience similar highs.
The UK Health Security Agency has issued Level 3 heat-health alerts across the south and east of England, the Midlands and London.
This requires health and social care workers to pay particular attention to high-risk groups of people such as the elderly and vulnerable.
Forecasters are predicting that the UK could face its hottest day ever next week.
The current record was in July 2019 when 38.7C was recorded at Cambridge Botanic Garden but as extreme heat builds across Sunday into Monday, next week could see that temperature being topped.
Wales saw its hottest day of the year on Monday with Cardiff’s Bute Park reaching 28.7C.
On Tuesday, a group of leading UK scientists suggested that heatwaves should be named similar to storms.
Heatwaves are becoming more likely and more extreme because of climate change.
The world has already warmed by about 1.1C since the industrial era began in the latter half of the 18th century, and some experts predict that temperatures will keep rising unless governments around the world make steep cuts to emissions.
In England, there were 2,500 excess deaths in the summer of 2020 as a result of hot weather, and the Red Cross predicts that heat-related deaths in the UK could treble in 30 years.