Nearly three inches of rain could fall over several hours in the worst-affected regions on Thursday.
The Met Office has issued three separate yellow rain warnings – one in western parts of Northern Ireland, one in northern England, and another for a large area covering most of England and Wales.
They warn of travel disruption, flooding, power cuts and road closures.
The highest rainfall totals are likely across the Pennines and North York Moors where 80-100mm could accumulate, the Met Office said.
Most areas are likely to see 20-30mm of rain, even increasing to 50-70mm.
There is also the chance of thunderstorms and strong winds in the southern half of the UK later in the day, forecasters predict.
Parts of the country saw more than the monthly average rainfall on Monday, with flash flooding damaging homes and disrupting travel.
There were further downpours on Wednesday evening.
Met Office forecaster Paul Gundersen said: “With the rain on Wednesday and Thursday potentially falling on already saturated ground a number of warnings for rain have been issued outlining the increased risks for potential impacts.”
The Environment Agency has 28 flood warnings in place across England, meaning flooding is expected.
There will be further outbreaks of rain in central and southern areas of the UK on Friday morning, the Met Office said.
The rain is expected to clear during Friday leaving conditions much colder on Saturday.
