The Met Office said there was an “80 per cent probability” of heatwave conditions between noon on Tuesday (30 June) and 6am on Thursday in parts of England.
Temperatures are set to soar as a swathe of roasting air sweeps in from northern France and Spain.
The forecast triggered a ‘Level 2’ alert with the Met Office warning: “Heatwaves can be dangerous, especially for the very young or very old or those with chronic disease.”
The Level 2 alert - calling for “alert and readiness” - is the third-highest warning that can be issued.
Graham Bickler of Public Health England said: “There is considerable evidence that heatwaves are dangerous and can kill.
“In the 2003 heatwave, there were 2,000 to 3,000 excess deaths [more than usual] in England. Across Europe, there were around 30,000 excess deaths.”
The Met Office said: “Temperatures are expected to build on Tuesday and into Wednesday, with the hottest day of the year so far expected on successive days this working week.”
Wednesday is likely to be the hottest day of the year so far, with temperatures soaring to 33C (91F).
A Met Office spokesman added temperatures should drop again on Thursday when severe thunderstorms are likely to hit central and western parts of the country, before climbing again ahead of the weekend.
Clive James, of St John Ambulance, added: “Extreme heat can be dangerous, particularly for the very young and old, but by being prepared you can spot the early warning signs and potentially be the difference between life and death in an emergency.
“Heat exhaustion and heat stroke are the most serious problems that can develop when the mercury soars so it’s essential that people can spot the signs, such as headache and dizziness, and get somewhere cool and rehydrated as soon as possible.”
AA spokesman Paul Watters advised motorists to carry water on long journeys and prepare for extreme temperatures to soften road surfaces.
He said: “Roads start to soften at about 27C and if the hot weather lasts they absorb the heat a bit like a storage heater.
“Road surfaces can become a bit like dough and this can lead to cracks and the surface of the road can start to give.”
The soaring temperatures could pose a risk to health, increasing the risk of dehydration, heat exhaustion and heat stroke.
Children, young babies and the elderly are particularly vulnerable.
Public Health England has issued advice on how people can stay safe in the high temperatures:
• Try to stay out of the sun between 11am and 3pm.
• Protect against sunburn and use on sunscreen of at least SPF15 with UVA protection.
• Wear sunglasses with UV protection to prevent damage to your eyes.
• Wear light, loose-fitting clothing and a hat to keep cool.
• Quench a thirst and drink plenty of cold fluids. If you feel dizzy or develop a headache try and rehydrate, using rehydration sachets from a pharmacy if necessary.
• Do not do too much exercise - use the hot weather as an excuse to take a day off.
• Never leave anyone or an animal in a closed, parked car.
• Keep your house airy - close curtains when the sun is shining and open windows during cooler parts of the day and at night. Turning off non-essential lights and electrical items will also help lower the temperature.
• Muslims observing Ramadan should take extra care and drink plenty of water between fasts.
• St John Ambulance advises anyone who feels unwell during the hot weather to get somewhere cool and rehydrated as soon as possible, and see a doctor if this does not help.