Video report by ITV News Correspondent Martha Fairlie
Britons are being urged to stay indoors until Friday and avoid the sun, as heatwave conditions are expected to reach up to 35C.
The Met Office has issued a heat-health watch alert for southern and eastern parts of England as temperatures look set to hit sizzling heights.
Authorities are advising people to "stay in doors for the strongest parts of the sunshine" and keep their homes as cool as possible.
When and where can we expect the hottest day to be?
Temperatures could soar to more than 34C (93.2F) in some parts of the country on Wednesday, beating the 33C (91.4F) recorded at Porthmadog in North Wales on June 28.
Met Office forecaster Alex Burkill said: "It does look likely it will be at least 33C at some point this week, most likely on Wednesday or Thursday, in the south east and east, London, Essex, Kent, east Anglia and into Lincolnshire – and that would make it the hottest day we have had so far.”
Temperatures are likely to be lower elsewhere.
What is a heat-health watch alert?
Heat Health Watch Service is designed to help healthcare professionals manage through periods of extreme temperature.
The service acts as an early warning system forewarning of periods of high temperatures, which may affect the health of the UK public.
When certain heat thresholds are passed, a warning is issued and sent to relevant health professionals and people working in social care.
What about those who have to work outdoors?
Workers such as builders and farmers are being advised to cover up and keep hydrated.
Public Health England are urging people to get sun-smart and take steps to protect their skin – such as using sun cream and wearing a cool shirt, or t-shirt, and a hat.
Top sun safe tips with Dr Monah Mansoori
Use at least factor 15 sunscreen in the sun and use plenty of it
Apply sunscreen to all exposed skin – don’t forget your neck and ears and your head if you have thinning or no hair
Wear sunglasses and a hat
Take particular care if have fair skin, moles or freckles, red or fair hair, or light-coloured eyes.