Salisbury and Southern Wiltshire are included in the area that is under the new Met Office Extreme Heat Weather Warning.
It advises staying out of the sun between 11:00 BST and 15:00 BST, drinking plenty of fluids and avoiding excess alcohol and physical exertion during the hottest parts of the day.
The more vulnerable are being warned of more adverse health effects.
It also warned that, as temperatures soar, some heat-sensitive systems and equipment may fail, leading to power cuts. There could also be delays to road, rail and air travel, meaning people experiencing long delays might suffer.
The Met Office said the high temperatures were expected in the day and at night, meaning restless nights for many.
The Met Office launched its new extreme heat warning at the start of June 2021 to highlight potential widespread disruption and adverse health effects. Amber is the second-highest level in the system.
It comes after a record-breaking number of heatwave deaths were recorded in England last summer.