Hikers and runners are among those being warned to take care as the UK is issued with its first amber weather warning.
The extreme heat is set to increase before it cools this week, according to the country's forecast service, the Met Office.
The amber weather warning means people, especially those that enjoy outdoors exercise and activities, should be vigilant to the dangers of the very high temperatures.
What is an amber weather warning?
Many countries have a system of weather alerts, especially for extreme conditions. The Met Office, which issues weather forecasts for England, Scotland, Ireland and Wales, has introduced a new-style extreme heat weather alert.
The amber warning covers all of south-west England and parts of southern and central England, as well as large parts of Wales. These areas will be on an amber alert until Thursday, when temperatures are expected to peak.
Over the weekend, all four nations recorded the hottest day of the year with temperatures of between 28.2C and 31.6C.
In many other countries, there have been warnings of heatwaves, too. For example, Spain has temperatures of up to 40C in many places. Nordic countries are also experiencing a heatwave, with Lapland recording the hottest day since 1914.
Earlier this month, Death Valley, which is located on the California and Nevada border, recorded the hottest month for more than a century. Much of western America and also western Canada saw very high and "unprecedented" temperatures at the end of June
Heatwave and extreme weather health risks
Adverse health effects for children and adults include heat exhaustion, dehydration and sun burn. Pets, especially dogs, can also be at risk, too.
Outdoors enthusiasts are at even greater risk. Over the weekend, six people sadly lost their lives in English lakes and rivers.
Hikers, runners and campers are warned to take extra care when exercising in the extreme heat and hot sunshine. Heading outdoors at cooler times of the day, such as earlier the morning and evening, makes good sense. Try to find shadier places to walk or run, such as in woodland, and make sure you are properly hydrated.
It is important to know how to run safely in hot weather and to heed tips on what to wear for hiking in the heat.
