A severe flood warning has been issued as forecasters said up to a month's rain could fall over 24 hours.
The Environment Agency has urged communities in the north and east of England, the Midlands and much of Wales to be prepared for flooding as two bands of very heavy rain were set to cross the UK on Friday and into Saturday.
The agency said the risk of flooding was expected to be the highest of a so-far wet year, with properties and transport in some parts of the country likely to be hit.
Yorkshire, the North East, the Midlands and East Anglia are likely to take the brunt of the severe weather, the Environment Agency said.
Craig Woolhouse, head of flood incident management at the Environment Agency, said: "We are expecting some very significant flooding in the coming days across large parts of the country and would strongly urge people to sign up to flood warnings, keep a close eye on the forecasts for their area and be prepared for flooding.
"We would also ask that people stay safe, by keeping out of dangerous flood water and not attempting to walk or drive through it.
"The Environment Agency has already opened incident rooms and has teams out on the ground checking on flood defences and clearing any blockages in order to reduce the risk of flooding as much as possible."
Nick Prebble, a forecaster with MeteoGroup, the Press Association's weather division, said one indication was that up to 100mm (3.9 inches) could fall over central and southern England in a 24 hour period but it was likely to be less.
He said: "There is quite a large area of rain edging in from south-east England and making its way westwards through the course of today. Much of England and Wales will be affected by rain."
The Met Office issued an amber weather warning - the second highest, and meaning "be prepared".
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