Forecasters have issued yellow severe weather warnings for thunderstorms across large swathes of the UK on Tuesday and Wednesday.
The heavy rain will bring a risk of flooding to some homes and businesses, along with road closures and cancellations of train and bus services.
"Heavy showers and thunderstorms will continue to spread north-east across England and Wales, while also increasing in frequency during the day,” Met Office forecasters warned.
"In a few places there is the potential for 20mm to 30mm of rain to fall in an hour and 30mm to 50mm of rain to fall in three hours.”
The UK usually sees about 78.1mm of rainfall in July on average, according to data collected by the Met Office between 1981 and 2010.
Hail is also possible in northern England and yellow weather warnings come with a "small chance of fast flowing or deep floodwater causing danger to life", the forecaster said.
The first warning, which will run until midnight on Tuesday, will cover almost all of England and Wales, excluding East Anglia and the southeast.
A second weather warning for thunderstorms on Wednesday will cover parts of Scotland, North Wales, north England and the Midlands.
The risk of sudden flooding will be a shock for many people after the record-breaking heatwave that swept across the country last week.
On Thursday, the UK saw its hottest day ever when a temperature of 38.7C was recorded at Cambridge Botanic Garden, beating the previous record of 38.5C set during a heatwave in 2003.
However, Marco Petagna, a Met Office meteorologist, said that the unsettled weather is not unusual for this time of year, although it will be a sharp contrast to recent weeks.
He added that the worst conditions are expected to pass after Wednesday.
“The trend through the week is for things to become less intense, but there are signs of some more unsettled weather coming in at the weekend,” Mr Petagna said.
Agencies contributed to this report
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