Christmas Day's clear skies will turn to cloud, wind and rain on December 26, with the south west of England experiencing the heaviest downpours.
The Environment Agency has issued 36 flood warnings - where flooding is expected - and 96 flood alerts - where flooding is possible.
Met Office forecaster Alex Burkill said a band of rain moving from the south west to the north east of England will ensure that "pretty much everywhere" will see some wet weather on Thursday.
He said: "For most places there is going to be a lot of cloud and some rain around too. The heaviest will be in south-west England and Wales.
"Because of the amount of rain we have seen this month there could also be some localised flooding."
About 30mm (1.2ins) of rain is expected to fall in some parts of south-west England, while other regions could see up to 20mm (0.8ins) throughout the day, Mr Burkill said.
Coastal areas in the South West are also forecast to be hit by winds of up to 50mph.
Despite the bad weather, temperatures are likely to be around 12C in the South and 5C further north.
Boxing Day shoppers in London will see the worst of the rain in the morning, before the weather clears later in the day.
Friday is forecast to be another wet day, but with temperatures increasing to 14C in the West and between 11C to 12C in the north east of England.
While the rain is forecast to clear later on Friday, it will return on Saturday to the north west of England. The South East, meanwhile, will be drier and brighter.