The Met Office has issued a yellow warning of rain which will last until just before midnight on Bank Holiday Monday.
The warning was previously issued from 6am until around 10am on Tuesday morning.
Although no flood warnings have been issued, rainfall is expected across much of England and Wales, with the possibility of it exceeding 30mm in some places.
Temperatures are expected to reach no higher than 14C - 10 degrees lower than the August Bank Holiday Monday in 2013.
Met Office spokesman Calum Maccoll said: "Temperatures are certainly down on last year with some rain at times, particularly on Monday. On Tuesday there will be a lot of low cloud in the morning but during the day we will start to see brighter conditions.
"This really sets us up for a bright day on Wednesday. We might see low cloud again but that should break up as the day progresses leading to sunny spells."
