The UK could get more than two months' rainfall in just a few hours as thunderstorms wash out the heatwave and sweep across the country.
Forecasters are preparing for a "worst-case scenario" of up to 150mm on Monday afternoon and evening in some parts..
The whole of the UK is now covered by a Met Office weather warning until Thursday evening.
The worst affected areas are expected to be between Birmingham and Cumbria, from around 4pm on Monday.
Brits are being warned isolated flooding is "possible" across parts of England, and agencies are monitoring the situation
Heavy rainfall was already reported across Devon and Cornwall in the south-west of England by early afternoon Monday, and forecasters have warned it will soon spread.
It follows an intense period of very hot weather which saw temperatures reach 34C at Herstmonceux in East Sussex on Sunday, the fourth consecutive day the mercury tipped over 30C in the south of England.
Despite the forecast for wet weather there is every chance the heat will linger, forecasters says.
Temperatures of up to 36C remain possible across the south coast at the beginning part of the week.
Bonnie Diamond, a meteorologist with the Met Office, said: "There is a pretty broad warning in place today, but there is another for central UK - the Midlands up to the North West of England to about Cumbria, and into Wales - which we are watching really closely.
"We are looking at the potential for some thunderstorms there from about 4pm to the early hours. "It's a worst-case scenario - a caution, really, for what could happen - but we are looking at a potential for 150mm rain in three or four hours.
"Normally for August you're looking at around 70mm for the month, so it's potentially a lot of rain.
"To get that much it will take some areas seeing frequent thunderstorms, and not just passing through."
Ms Diamond said there had been some thunderstorms in the south-west of England on Monday morning, while temperatures across the country remain humid, particularly in the south-east of England.
She said: "Things can change at short notice, and the thunderstorms can come on quite quickly, so people may want to monitor the up-to-date forecasts and our Twitter feed to make sure they don't get caught out in it."
The far north of Scotland is the only part of the UK not currently covered by Monday's broad weather warning.
Northern Ireland is also affected from Monday until Wednesday.
The Environment Agency said it would update its flood warnings and alerts as the situation changes, and said those wanting to keep up to date can check on its website or social media, or through the Floodline service on 0345 988 1188.
The Met Office warned that flash flooding could cause travel disruption and power cuts, but also cautioned about the risks caused by fast flowing or deep floodwater.
HM Coastguard dealt with 340 incidents across the whole of the UK on Saturday during the weekend's scorcher.
It was the highest number of call-outs in a single day for well over four years.
There were a further 335 incidents on Sunday.
A 12-year-old girl died after going missing in the River Leven, near Balloch Bridge, Loch Lomond, in the west of Scotland on Sunday evening. Her body was discovered by emergency services hours later.
In Norfolk, a woman in her 30s died after getting into difficulties in the sea at Waxham on Sunday.
