The Met Office has earlier alerted Brits of a scorching heat wave coming this weekend.
The warning was initially issued for a time period from 9am on Friday to 9am on Monday but has now been extended to 9am on Tuesday.
Health authorities plead with people to take precautions as the temperatures are expected to soar as high as 30C, marking the hottest days of this year.
While some parts of the UK will be hotter than Marbella, Ibiza and Tenerife as a 'plume of warm air' moves in from the South, there is a growing concern of thundery showers following the heatwave.
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This weekend is expected to see the hottest day of the year with record-high temperatures hitting London and other areas of England.
The warning by the UK Health Security Agency and the Met Office, initially issued from 9am on Friday to 9am on Monday has now been extended to 9am on Tuesday.
The amber alert - when the heat is likely to impact the wider population, not just the most vulnerable - concerns five regions: West and East Midlands, East of England, South East and South West.
Yellow alert - when the weather is likely to impact the vulnerable with underlying health conditions, or the elderly - is in place for North East, North West, Yorkshire and Humber and London.
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The health authorities ask people to take precautions and remind them of the implications of hot weather on their health.
The warning also comes amid fears of thunderstorms triggered by the scorching heat wave from the South.
Some areas of England could see more than 60mm (2 inches) of rainfall in just a few hours on Sunday.
A yellow warning for thunderstorms is in place for much of the western side of the UK between noon and 9pm.
It also covers Wales and south and western England between 2pm and 9pm on Saturday.
According to the Met Office, the thundery showers may continue into Monday, particularly in southern and central England as well as in Wales.
Forecasters have warned of the potential for fresh flooding.
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Sources used:
- Sky News: 'UK weather: First heat-health alert of year extended into next week'