Having come into effect at midnight today (Thursday 11 August), the amber warning applies to the southern half of England and parts of eastern Wales and will be in force until Sunday night.
It means that adverse health effects are ‘likely to be experienced by those vulnerable to extreme heat’, while the wider population may experience sunburn or heat exhaustion – including dehydration, nausea and fatigue - and other heat-related illnesses.
Delays to road, rail and air travel are also possible, with some changes in working practices and daily routines likely to be required, and an increased chance that some heat-sensitive systems and equipment may fail.
The Met Office also said there is an increased risk of water safety and fire-related incidents as more people are likely to visit coastal areas, lakes, rivers and other beauty spots.
Met Office Deputy Chief Meteorologist Dan Rudman said: "Thanks to persistent high pressure over the UK, temperatures will be rising day-on-day through this week and an Extreme heat warning has been issued.
“Temperatures are expected to peak at 35°C on Friday and Saturday, or even an isolated 36°C on Saturday. Elsewhere will see temperatures widely into the high 20s and low 30s Celsius.
“Coupled with the high daytime temperatures there will be some warm nights, with temperatures expected not to drop below the low 20s Celsius for some areas in the south.”
The new amber warning follows Britain's first ever red extreme heat warning last month.
BBC Weather forecaster Matt Taylor said this heatwave 'won't be as high' as the previous one, but that it will be 'much more prolonged'.
"We've already seen temperatures above 30°C every day this week - Monday, Tuesday and Wednesday - and that will continue until Sunday," he said, adding that temperatures could reach 35°C and peak at 37°C in some areas between the Midlands and London, while Cardiff in Wales may see temperatures exceeding 30°C.
National Highways Head of Road Safety, Jeremy Phillips said: “It is always very important to plan ahead for your journey and this advice remains the same during periods of hot weather.
"When hot weather is forecast, please remember to take plenty of drinking water with you – enough for you and your passengers. You can visit our website to find out more information about travelling during hot weather.
“We also advise everyone should check their vehicles, such as tyres, coolant and oil levels, before heading out.”
Dr Justine Shotton, President of British Veterinary Association, also urged people not to forget to give 'extra attention' to pets, many of whom may struggle in the hot weather.
"Animals need extra care during the summer to keep them safe from heat-related illnesses such as heatstroke, heart conditions, breathing difficulties and sunburn, many of which can sadly be fatal," she said.
“Make sure animals have access to fresh drinking water, good ventilation and shade from direct sunlight at all times.
"Dogs especially can overheat easily, so make sure they aren’t walked or exercised in the hottest parts of the day or left inside a hot car or conservatory for even a little while.
"Keep an eye out for early signs of heatstroke, such as heavy panting, drooling, restlessness, and lack of coordination and contact a vet immediately in case you have any concerns.”