Some places have suffered more than a month of rainfall in 24 hours, says the BBC Weather Centre.
Peter Gibbs, who is a BBC and Met Office weather forecaster, said South Wales and parts of south west England were the worst affected areas.
Chris Mills, of the Environment Agency, said people based in areas where there are flood warnings "need to make sure that they are looking after their property, that they are alert [and] that they are ready to leave their property if necessary".
"Don't go anywhere near the water, don't try to cross the water in a vehicle, even by a 4x4, and remain alert,'' he said.
BBC forecasters said 40mm of rain had fallen in Caerphilly in south Wales and on Exmoor.
And forecasters are warning of a very heavy period of rain continuing for up to 24 hours, affecting northern England, south-eastern Scotland and Northern Ireland.
The heavy rain will continue through the night and well into Saturday in these areas.
The weather has caused disruption across much of Wales and England:
• A major incident has been declared in Bridgend due to rising water levels in the River Ogmore. The council is evacuating staff from its civic offices and businesses are being advised about the risk of flooding
• The Jazz in the Park event in Pontypool has been cancelled, as was Cardiff Pride 2008
• A number of roads are under water in Gloucestershire and a road in Crumlin in Monmouthshire was closed following a landslide
• Gusts of more than 60mph have been recorded at headlands near Torbay in Devon
• Arriva Trains is advising people in south east Wales not to travel by train as all rail services are severely disrupted by flooding
South Wales Fire Service said it had received 350 flood-related calls in six hours on Friday, compared with a normal daily total of about 130.
Fire crews and council workers were called to homes in the Lydney area of Gloucestershire on Thursday after flash floods.
Flooding caused substantial problems throughout that county in July last year.
The ground is saturated after an unusually wet August, increasing the risk of flooding across the UK, experts said.
"We are expecting 20mm to 40mm of rainfall in the next 24 hours on already saturated ground, which is the biggest problem. We've had a lot of rain already," said BBC weather forecaster Darren Bett.
Flooded roads and stranded motorists in England
Flooding is likely to disrupt businesses in the worst affected areas.
Tina Beeston, the publican of the Market Tavern in Tenbury Wells, told the BBC her business was flooded last year and there was a serious risk that it would be again.
"There's probably eight premises that are already flooded, so these are businesses that have only just picked themselves back up again. It's looking pretty grim for the businesses in Tenbury Wells," she said.
Sarah Macdonald, who also lives in the town, echoed these fears.
She said: "The brook can't cope. The defences they're working on at the moment are absolutely underwater. Here we go again. The fourth time in less than a year and a bit."
The AA has reported a higher than normal number of breakdowns with cars getting in trouble after driving through floodwater.
Commuters have been advised to be careful on the roads and to leave extra time for journeys.
The public can keep updated through news outlets and by calling the Floodline on 0845 988 1188.
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