Parts of northern England and Scotland are facing flood fears, with heavy rain expected to sweep across most of the country, forecasters said.
Heavy downpours have been forecast to hit southern and central England and Wales today before moving north, with as much as 60mm (2.4in) of rain predicted in Cumbria and areas of southern Scotland by Friday.
MeteoGroup, the weather division of the Press Association, warned of the prospect of flooding in some areas and said there was "no end in sight" for the wet weather.
It follows the wettest April since records began, dating back a century to 1910.
Forecaster Brendan Jones said: "Very few places south of Manchester will miss the wet weather today.
"The rain will continue to move north and intensify on Thursday and into Friday. There is the possibility of local flooding problems in southern Scotland and areas of northern England such as Cumbria.
"Most of the country will see substantial rain over the next few days. Unfortunately there is no end in sign for the unsettled weather."
Between 20 and 40mm of rain are set to fall in most parts of England, Wales and southern Scotland over the next two days, Mr Jones said.
The predicted downpours follow a period of unsettled weather which has seen unusual storms hit parts of the country.
A "supercell" storm travelled through the south Midlands earlier this week, bringing rain, large hailstones, and a tornado in Oxfordshire.
The thunderstorm started in Wiltshire, and moved across Oxfordshire - where a tornado was reported in several places including Bicester, Eynsham, and Kidlington - then moved to Buckinghamshire.
The Met Office confirmed the UK had seen its wettest April on record, with almost five inches (126.5mm) falling on average during the month.
The amount was almost double the long term average for April of 69.6mm (2.7 inches) and outstripping the previous record set in 2000 by around 0.24 inches (6mm).