The Met Office upgraded its yellow warning to amber for the east and south east of England after more than a month’s rain fell in just nine hours – and now the storm clouds are heading north.
On average, 51.6mm of rain falls throughout the entire month of August but more than that arrived in just one burst.
An Environment Agency spokesman said the heavy and thundery rain that will spread across many parts of England has the potential to cause isolated incidents of localised surface water flooding.
He added: “The Environment Agency is monitoring the situation closely and supporting local authorities who will respond to any reports of surface water flooding.
“Our teams are out on the ground working hard with partners to reduce the risk of flooding, and will issue flood alerts and warnings for rivers if required.
“The public, as always, are urged not to drive through flood water and if away on holiday in England, check the flood risk for that area on the GOV.UK website.”
The RAC has also issued safety advice for drivers contending with flooded roads.
RAC spokesman Pete Williams said: “The RAC is urging drivers to expect the worst and prepare for challenging conditions on the roads.
“Drivers are likely to get a taste of some tough driving conditions over the next 24 to 48 hours with the forecast of torrential rain and the risk of hailstones.
“Exercising care and caution is vital in heavy rain conditions, especially as traffic volumes are particularly high at the moment.
“Cut your speed, use dipped headlights and avoid driving through deep water if you encounter flood conditions and remember to show consideration for other road users, particularly cyclists and pedestrians. Also be prepared to pull over and wait for the worst conditions to subside if you are hit by a heavy storm.
“It could be wise to delay your journey until conditions improve and listen out for the latest weather and traffic news on the radio.”