The Environment 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.
The south west is expected to see the worst of the rain on Friday night and Saturday morning, while at the other end of the country, Northumberland is expected to see heavy rain and flooding on Friday night.
The agency says 47 flood warnings (notice that flooding is imminent) are in force while 123 flood alerts (flooding is possible) have been issued across Wales and the north, south and south western and eastern counties of England.
Craig Woolhouse, head of flood incident management at the Environment Agency, said: “We would urge people to remain on alert for flooding, especially in Devon, Cornwall and Somerset. It has been raining heavily today, and the situation could escalate quite quickly.
“We would encourage everyone to keep up to date with the latest information and flood warnings by checking the Environment Agency website and Twitter feed. When flood warnings are issued, residents in those areas should think about moving valuable possessions to safety. Our Floodline service on 0845 9881188 can also provide advice.
“We would also ask people to remember to stay away from flood water, and do not walk or drive through it, as it is often fast moving and can contain sewage and other debris.”
The Prince of Wales’s visit to Hebden Bridge, which was badly hit by the floods which devastated parts of West Yorkshire and Lancashire two weeks ago, was delayed by the weather and he faced heavy rain as he toured the town.
Earlier this week, Channel 4 weather presenter Liam Dutton warned that the UK has not seen the last of flash flooding.
In his weather blog he wrote: “Most of the time, low pressure systems move through giving a spell of rain, followed by drier weather. However, during the next few days an area of low pressure will move over the UK and get stuck.
“When low pressure is present, air is more prone to rising in the atmosphere and as a result water vapour condenses to produce cloud and rain.”