An urgent amber health alert was issued for parts of England this week.
The UK Health Security Agency (UKHSA), said the nation was set for a "enhanced hot weather response".
A yellow weather alert has been issued for the rest of the country.
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The warnings will come into place from 9am on Tuesday, until 6pm on Wednesday.
A heatwave is declared when a location has to hit or exceed 25C in the west and north and 28C in London for three consecutive days | PAThe East Midlands, West Midlands, East of England, London, and the South East all fall under the amber alert.
The yellow alert covers the North East, North West, Yorkshire, The Humber, and the South West.
Scotland, Wales and Northern Ireland are exempt from the alerts.
Areas including east Wales, the Midlands, and south-eastern England are expected to experience the most intense heat, with temperatures reaching 34C.
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An urgent amber health alert was issued for parts of England this week
|UKHSA weather alert
Met Office Deputy Chief Meteorologist Tom Crabtree said: "Warmth is the focus in the forecast in the first half of this week, with temperatures likely to peak on Tuesday around the mid-30s, but remaining above average in the second half of the week, particularly further to the southeast.
"The exception to the widely warm conditions will be northwest Scotland on Monday, where showers will be more frequent.
"Warmth will develop there from Tuesday with temperatures peaking in the mid to high 20s."
A combination of high pressure moving across southern Britain and low pressure in the Atlantic, which contains what remains of Tropical Storm Dexter, will pull warm air northward from the south.
Areas including east Wales, the Midlands, and south-eastern England are expected to experience the most intense heat
|WXCHARTS
Today temperatures will be highest in the late afternoon, predicted to reach 30C.
Most of England and Wales will experience temperatures in the upper twenties.
On Tuesday, most of England is predicted to exceed 30C.
Coastal areas will be slightly cooler.
Heavy downpours may form and drift northward across certain regions as the week progresses.