Parts of the South East have seen up to a month’s worth of rainfall in one day, the Met Office has said.
Hampshire and Buckinghamshire have seen heavy rain over the past 24 hours while Bedfordshire has also had wet weather.
The areas particularly affected were Otterbourne, which saw 67mm of rainfall from 11pm Thursday to 5pm Friday, and Dancers End, Drayton Parslow, Toddington and southern Farnborough, which had rainfall between 65mm and 57mm.
The Met Office said temperatures have dropped across the UK following sweltering weather earlier this week, and torrential downpours that could cause flooding are expected in some southern areas.
The Environment Agency has issued 39 flood alerts, where flooding is possible, in London and surrounding areas, Kent, parts of the Home Counties, the West Midlands and Wiltshire.
A yellow weather warning for rain covering much of England, apart from Devon, Cornwall and the North West, has also been issued by the Met Office.
On Saturday, temperatures will range from highs of 19C (66.2F) in London and Cardiff to 18C (64.4F) in Belfast and 17C (62.6) in Edinburgh.
This is due to the mercury being set to rise a couple of degrees everywhere on Saturday and remaining stable on Sunday.
Met Office spokesman Grahame Madge said: “In the South East, there will be rain every day for the next few days, and some places could see up to 80mm (3.2in) on Friday.
“The heaviest rain will be in places from Hampshire up to Yorkshire, where we are quite likely to see intense rainfall which will clear away tonight.”
He added that Saturday has the “potential to be drier” in the South but the deluge is set to return on Sunday and last into next week.
Saturday is predicted by the Met Office to have some sunshine across most of the UK with the odd shower across Scotland and south England.
Sunday meanwhile could be “unsettled” across the South, with spells of heavy and thundery rain in England.
The Environment Agency has urged drivers not to plough through flood water, warning that just 1ft (30cm) of flowing water is enough to float a vehicle.
Surrey Fire and Rescue Service has also alerted drivers to “lots of standing water” on the roads, while the Driver and Vehicle Standards Agency (DVLA) has issued advice on driving safely through summer showers.
The DVLA’s tips include allowing at least double the usual separation distance between your vehicle and the one in front, keeping speed down and using dipped headlights so that other drivers can see you.