People yet to depart for Glastonbury have been urged to delay their journey, while those already on their way have been told to stop somewhere "safe and warm" after weeks of wet weather led to massive delays getting on to the festival site.
On Twitter, people hoping to reach Worthy Farm in Somerset complained of being stuck in queues for up to 12 hours.
Festival organisers said "wet weather and ground conditions" were causing "heavy traffic congestion" around the site.
"If you are coming to the festival by car or campervan/caravan and have yet to begin your journey, please do not set off," a statement said.
"If you have set off and have yet to reach the site, please stop somewhere safe and warm (ie a service station). Grab some essentials, as you may have a long wait in your vehicle."
Gates officially opened at 8am today, two days before the festival entertainment starts proper, but festival founder Michael Eavis said people had ignored advice not to arrive early.
"We did ask people to come in later," he told BBC News.
Queuing for hours took its toll on some people.
At least some people appreciated the irony of this referendum-themed photo.
People who did make it on to the site quickly got stuck into the mud.
Glorious, glorious mud.
Mud as far as the eye can see.
There is still mud.
The headliners are unchanged.
Matt Tucker is the UK Picture Editor for BuzzFeed and is based in London.
Contact Matthew Tucker at firstname.lastname@example.org.
Matthew Champion is a weekend editor for BuzzFeed News and is based in London.
Contact Matthew Champion at email@example.com.
Got a confidential tip? Submit it here.