Health warnings have been issued ahead of what could be the hottest September day in more than 50 years.
Britain's Indian summer will sizzle with temperatures on Tuesday potentially reaching 31C (88F), with the best chance of that scorching high in London, the Met Office said.
The hot weather will hit the East of England, the South East, the capital and the East Midlands, which will all be put on "heatwave Level 2 status" from Monday evening.
Following the balmy forecasts, warnings have been sounded by Public Health England (PHE) urging caution over the coming days and nights.
Dr Thomas Waite, from the extreme events team at PHE, said: "Because the heat is going to arrive very soon, think today about what you can do, and for those around you, to stay cool during the daytime and particularly at night.
"Much of the advice on beating the heat is common sense and for most people there's nothing to really worry about.
"But for some people, such as older people, those with underlying health conditions and those with young children, summer heat can bring real health risks.
"To keep homes and sleeping areas cool at night remember to close curtains on windows that face the sun during the day, once the sun is off windows open them up to get a breeze and think about turning off electrical devices all over the home as they can generate unwanted heat too."
The Met Office declared a Level 2 heat-health alert on Monday morning - which means there is a high chance that temperatures will hit certain temperature thresholds for at least two days and the intervening night.
The high temperatures predicted means that Britain could be as warm as Bangkok in Thailand, and hotter than forecasts for Madrid and Los Angeles.
The last time temperatures soared above 30C (86F) in September was in 2006 in Kew Gardens, which hit 30.5C (87F) on September 11.
If the mercury rises above 31.6C (88.9F), which was reached at Gatwick on September 2 1961, then it will be the hottest day for 55 years.
The highest September temperature recorded was in 1906 when the mercury hit 35.6C (96.1F) in Bawtry, South Yorkshire.
Most of England will bask in temperatures in the high 20s, while Scotland will also enjoy the warmth with Aberdeen and Glasgow possibly seeing 20C (68F) to 23C (73.4F), and there is a chance Aviemore could hit 24C (75.2F).
Dr Waite added: "The hot weather won't make life difficult for all of us; indeed, many of us will make the most of it when the sun shines. But some people may not be able to adapt to the extra strain hot weather will put on their bodies and may feel the ill-effects.
"Each year we hear stories of people who have fallen seriously ill because, even though it's hotter, they may wear clothes which are too warm for hot weather, they may not drink enough or just try to do too much."