More than a month's rain is expected to fall over just two days in parts of the UK this week.
A severe weather warning has been issued by the Met Office, with fears of flooding and disruption tomorrow and Saturday.
Central and northern England is expected to be worst affected, with an amber alert - the second most severe category that can be given - in place for north-east England.
Up to 100mm (3.9 inches) of rain could fall in 36 hours during the downpours. The average UK rainfall for July is 69.9mm (2.8 inches), and 64.4mm (2.5 inches) for the north of England.
And thundery showers are expected to affect large parts of eastern, central and northern England today.
Met Office chief forecaster Martin Young said: "We are expecting outbreaks of heavy rain across a wide area of the UK, with worst affected spots likely to be in central and northern parts of the country.
"Rainfall totals could be 40-60mm (1.6-2.4 inches) widely across warning areas, but some places could see up to 100mm (3.9 inches) of rain through Friday and into Saturday.
"Given the saturated ground from the record rainfall in June, this could cause disruption - including difficult driving conditions and flooding in some areas."
People living in the affected areas have been warned to be prepared for flooding.
An Environment Agency spokesman said: "Heavy rain on Friday and Saturday may lead to significant flooding of properties across parts of northern England.
"We urge people to remain vigilant and prepared for flooding by checking the Environment Agency website and signing up for localised river flood warnings.
"The Environment Agency has mobilised teams across the country to check flood defences, clear any river blockages and closely monitor river levels to help reduce the risk flooding."
Less severe yellow warnings have been issued for much of England and Wales and parts of southern Scotland for tomorrow.
The deluge could affect the men's semi-finals at Wimbledon.
While tomorrow's downpours will be the most widespread, the Met Office has also issued yellow weather warnings for some areas of the UK for today.
The heaviest rain today is expected in the north-east and central areas of England, with some some thunder punctuating the downpours.
A Met Office spokesman added: "Up to an inch or so of rain could fall in a relatively short period of time, but in other areas they won't see much rain at all and a good proportion will see some sun. It won't be a complete washout.
"Where rain does happen today, there could be torrential downpours."
Last month was the wettest June since records began, with double the average rain falling during the month. Provisional figures from the Met Office showed the UK received 145.3mm (5.7 inches) during June, beating the previous record of 136.2mm (5.4 inches) in June 2007.
It was the second month this year to see record-breaking amounts of rain, after this April became the wettest in the records dating back more than a century to 1910. June saw long, prolonged rainfall and short but exceptionally heavy showers, and ended with freak storms which battered areas of the Midlands and the North East. The exceptional amount of rain caused floods in Wales and parts of England.
In Northern Ireland parts of Co Down were flooded by torrential rain. Down District Council said it had declared an emergency in the area.
Newcastle SDLP councillor Carmel O'Boyle said flood water was "several feet deep" in some areas of the town.
Northern Ireland Fire and Rescue Service rescued a woman from her car who was trapped in flood water on the Burrenbridge Road in Castlewellan, Co Down. The water level is beginning to subside.
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