The band of rain stems from a complex area of low pressure, expected to deepen over the Bay of Biscay later today, before pushing north towards the UK.
The rain will arrive into southern areas Wednesday morning, before spreading north through Wednesday afternoon and into Thursday before clearing during the day.
A Met Office spokesman said: “Persistent and at times heavy rain is expected to spread from south to north during Wednesday, this affecting much of England and Wales.
“Some heavy and thundery showers could also develop across East Anglia and southeast England during the afternoon. Strong and gusty winds are also expected, with gales in exposure.
“Rain should ease from the southwest later on Wednesday, but will persist across the north of the warning area into Thursday.
“Some localised flooding is possible from fast responding water courses and standing water, this perhaps disrupting travel
“Most parts of the warning area should see 15-30 mm of rain, however some places could see as much as 40-70 mm.”