A four-day yellow weather warning for thunderstorms is in place but temperatures could still reach a scorching 37C.
Many in the UK basked in fine and dry weather with temperatures in the mid-30s over the weekend – but torrential rain is now being forecast.
A 80mm deluge – equating to a month’s rain in a few hours – could cause hazardous flash flooding, according to the Met Office.
The national forecaster says: “Some places are likely to see severe thunderstorms through Monday afternoon and evening – but there is uncertainty in location and timing.”
The heat is set to be maintained, however, but the humidity and change in pressure is ushering in stormy weather as August moves towards its mid point.
Forecasters say Monday could be the start of the joint-longest extreme heatwave on record – with at least 35C a possibility three days running.
Monday morning is set for temperatures in the teens, with thunder having already moved into parts of the country overnight, while hail and strong winds are also due.
BBC forecaster Helen Willetts said: “That heat eventually eases by the end of this coming week, but with the heat and humidity there’s a growing risk of thunderstorms – there are warnings out…also because we’ve got this low pressure…migrating further northwards.
“So a gradual process, whereby the atmosphere turns more unstable and with it will unleash some rather large thunderstorms or longer spells of thundery rain.”
She said on Monday it will be scattered storms moving from south to north but still with an “abundance of sunshine” with temperatures leading to an “oppressively hot day” in southern and eastern areas.
“It will be warm in the sunshine further north, but clearly once one of those showers comes along they will be torrential and they won’t ease by the end of the day – they’ve got their energy from the day’s heat.”
Overnight into Tuesday will be another uncomfortable one with thick humidity and cloud coverage, not dropping below 20C in London.
“That’s really quite high for a nighttime temperature,” Ms Willetts added.
The Scottish Environment Protection Agency has issued two flood alerts for Monday in the Scottish Borders and Dumfries and Galloway.
UK 5 day weather forecast
Some sunshine for most and a good deal of dry weather in eastern areas, but some thunderstorms developing, most frequent across western parts of England and for Wales. Southern UK very hot again.
A warm night in prospect for most areas, especially southern UK. Areas of thunderstorms moving north, with clear spells and a few fog patches later.
Sunny spells for most areas, and although some places will remain dry, scattered thunderstorms again developing. Very hot in the south.
Outlook for Wednesday to Friday:
Very warm and humid with some heavy showers and thunderstorms for many parts, but with sunny spells too. Temperatures likely becoming less warm by Friday.