A North/South weather divide is set to emerge as thunderstorms hit the North while the South basks in hot summery sunshine.
Health warnings were issued ahead of what could be the hottest September day in more than 50 years, but “yellow warnings” of rain are now in place for Northern England and South East Scotland.
Britain’s Indian summer is set to sizzle on Tuesday and temperatures could reach 31C (88F), with the best chance of that scorching high in London, the Met Office said.
There will be different scenes further north as outbreaks of rain may turn torrential and thundery, with as much as 30mm (1 inch) in some places, possibly causing flash flooding and disruption to travel.
The Met Office said: “A very warm and humid air mass is in place across much of the UK, with conditions marginal for thunderstorm development this afternoon as temperatures rise.
“Northern England and southeast Scotland are currently judged the most likely area to see scattered thundery downpours but even here many places will have no more than light rain.
“However, where storms do form, there is a lot of moisture and energy available, with the risk of more than 30mm of rain in around an hour, large hail and frequent lightning.”
The yellow warnings are valid from 2pm to 11pm.
The hot weather will hit the East of England, the South East, the capital and the East Midlands, which was 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, and pointing out the risks to older people, those with underlying health conditions and people with young children.
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 parts of 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).