Spring showers and longer spells of rain will characterise the weekend, especially in southern England, although there will be some sunshine late on Monday.
A spokesman for the London Weather Centre said: "The rain over the weekend won't be particularly bad and the weather will improve as the weekend proceeds.
"However there will be a number of showers and longer spells of rain across the country, especially on Saturday.
"Monday afternoon will be mainly dry, with some sunshine coming through later in the day."
Average temperatures will be 14-15C in the north and 16-17C in the south with westerly winds described as "breezy".
The National Rivers Authority reported one flood alert in west Wales after several hours of rain yesterday and overnight.
The RAC advised drivers to steer clear of the junction of the M4 and M5 at Bristol, the junction of the M5 and M6 in Birmingham and junctions 20-21A of the M6, which are all expected to be busy this weekend. A spokesman said: "All the signs at the moment are that we're in for a particularly busy bank holiday which has been dictated by the bad weather.
"Although work will be suspended, most major roadworks will remain in place and that can only add to the expected jams."
Eurostar said it was expecting a record number of passengers on Monday, possibly beating its current 24-hour record of 20,000 travellers set on Easter Monday.