Thousands of music fans have been advised to don their wellies as the Reading and Leeds festivals get under way in wet conditions.
River levels have even been lowered near the Reading site amid fears of flooding.
The three-day sister festivals are set to see performances from headliners My Chemical Romance, Muse and The Strokes.
Other acts taking part across the weekend include Pulp, Elbow, Thirty Seconds to Mars, Pete Doherty, Frank Turner, The Streets and Liam Gallagher's band Beady Eye.
Rain has been falling on both sites throughout the day - with more expected on Saturday. There is a brief respite on Sunday however, when the sun is finally expected to come out.
The Environment Agency has lowered the water levels near Reading Festival by about six inches by opening weirs downstream of the site, because the fields that host the festival have poor drainage.
An Environment Agency spokesman said: "This cannot prevent the site flooding, but it does lower the risk following heavy rainfall that the site will flood by helping to increase the drainage.
"This action has been taken following discussions with the festival organisers and, because we have had time to reduce the levels over a number of days, has not increased the flood risk upstream or downstream."
Victoria Kettley, a forecaster with MeteoGroup, the weather division of the Press Association, said: "There has been quite a lot of rain around already so things could get muddy.
"It looks like there will be showers across the morning in Reading tomorrow. It will be largely dry but if there are showers, they will be quite heavy. Leeds also has a risk of getting more rain. Sunday currently looks a bit better at both sites, and there will be decent periods of sunshine."