Forecasters warned of torrential downpours tomorrow for the first day of the Bank Holiday weekend.
The pop festivals in Reading and Leeds are among locations at risk of a soaking with up to an inch of rain predicted in some places.
Andy Ratcliffe, a forecaster with MeteoGroup, the weather division of the Press Association, said: "There are going to be widespread and frequent heavy showers and thunderstorms, with torrential rain in some places.
"We are looking at 5-15mm (0.6in) generally over the 24 hours from midnight, and perhaps locally up to 25mm (0.98in) in some areas. If there are torrential downpours, with a lot of rain falling in a short space of time, there could be local flooding."
Today saw rain in the south west from early this morning, which spread to other parts of England including along the south coast, with downpours in Sussex and elsewhere.
After tomorrow's heavy rain, Sunday is looking drier, though with a risk of showers in northern and eastern areas, before rain returns from the west early on Monday, spreading to most areas, though the south east might escape some of the worst of the conditions.
Temperatures are not expected to reach much higher than 20C (68F) over the weekend - a stark contrast with last weekend when the thermometer touched 32C (90F).
Around 1.8 million Britons are heading off abroad for the break. Travel organisation Abta said Spain and its islands were the most popular overseas destinations for Britons this weekend.
Paris, Amsterdam and Dublin were among the most popular city breaks, while domestic tour operators reported the south and south west coasts of England were favourite spots for those taking breaks in the UK.
An estimated 400,000 people will fly from Heathrow over the weekend, with 250,000 leaving from Gatwick, 150,000 from Manchester, 124,000 from Stansted, 68,500 from Luton and 62,500 from Birmingham.
Channel Tunnel high-speed train company Eurostar will carry 60,000 departing passengers this weekend and ferry services and Eurotunnel trains will be busy.
On the railways there will be some engineering work but 94 per cent of services will run as normal, with replacement bus services reduced by 58.5 per cent compared with last August bank holiday.
Network Rail and the Association of Train Operating Companies said about 6.1 million passengers are expected to travel on main line trains between Saturday and Monday.