People travelling to Glastonbury by car, caravan or campervan have been warned not to travel because "wet weather and ground conditions" are causing congestion.
Festival organisers urged people who had set off but not yet reached the site to "stop somewhere safe and warm".
"If you are already queueing to get into the site, please bear with us and follow the instructions from the signage and traffic managers," they advised in a Twitter message.
People arriving by coach or train were told to travel as planned but to be prepared for "your journey to take longer than usual".
Wednesday will see the start of arrivals at the event, with more than 180,000 festival-goers expected to pour in over the rest of the week.
The Met Office posted a weather update on its Twitter account, saying there were further outbreaks of rain over Glastonbury on Wednesday morning.
The festival site was dubbed a mudbath earlier in the week following showers, and pictures shared by workers at the festival showed boggy fields and submerged paths.
This year's headline acts are Muse, Adele and Coldplay, who will all take to the famous Pyramid Stage.