Parts of the rest of the UK could reach 33C as the day progresses, which would be hotter than some of Europe's other top holiday destinations – including Tenerife.
On Monday the Met Office issued its first-ever amber warning for "extreme heat".
It came into force shortly after 4pm and will remain in place until midnight on Thursday.
The warning covers London, South East England, South West England, Wales and parts of the Midlands.
The alert warns that people could experience some adverse health effects including sunburn or heat exhaustion and other heat-related illnesses.
It is also possible that some heat-sensitive systems and equipment may fail, leading to power cuts and the loss of other services to some homes and businesses, the Met says.
Forecasters added there could be "some delays to road, rail and air travel".
A yellow weather warning for thunderstorms is also in place for most of Tuesday and covers parts of the East Midlands, East of England, London and South East England as well as the West Midlands.
Thunderstorms may bring torrential downpours to some areas, with the hardest hit places seeing up to 60mm of rain in just two hours.
Forecasters have warned people to stay out of the sun during the afternoon, close their curtains to keep living rooms cool, stay out of direct sunlight and drink water.
Public Health England also issued a heat-health alert, warning members of the public to take measures to stay cool and look out for vulnerable people.
Marco Petagna, a Met Office forecaster, said: "We have an amber extreme heat warning in place until Thursday.
"Temperatures are likely to peak at around 32C, even reaching 33C in isolated spots across southern England, so very warm for many.
"Even the more cooler areas will see temperatures in the twenties while other places could see some thundery showers."
Mr Petagna put the warm weather down to an area of high pressure above the UK.
He added: "I'd say people should stay out of the midday sun, apply sunscreen when outside, be wary of any health issues you may have and make sure to stay hydrated.
"As this heat remains in place, people will be vulnerable to dehydration, fatigue and nausea, so make sure to stay safe on public transport and if you are heading to the beach."
Additional reporting by Press Association
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