Environmentally-damaging levels of illegal drugs have been detected in a river running through the Glastonbury festival site, scientists have said.
Samples taken before, during and after the festival in 2019 were found to have levels of MDMA and cocaine so high it could be damaging to wildlife further downstream - including rare eels.
Festival organisers say they have their own sampling regime agreed with the Environment Agency, who raised no concerns after the pre-pandemic festival.
It is thought the drugs entered nearby rivers through public urination on the site.
Experts are now urging festival-goers to use the official toilets provided by organisers to avoid this happening.
The samples were taken from the Whitelake River both upstream and downstream of the festival site.
The study found MDMA concentrations quadrupled the week after the festival which suggests long-term release from the site.
Scientists also discovered cocaine concentrations rose to levels known to affect the lifecycle of European eels - a protected species.
'Festivals are an annual source of illicit drug release'
Dan Aberg, who is a masters student at Bangor University, worked with Dr Daniel Chaplin from the Centre for Environmental Biotechnology (CEB) on the study.
He said: "Illicit drug contamination from public urination happens at every music festival.
"The level of release is unknown, but festivals undoubtedly are an annual source of illicit drug release."
He added: "Unfortunately, Glastonbury Festival's close proximity to a river results in any drugs released by festival attendees having little time to degrade in the soil before entering the fragile freshwater ecosystem."
The researchers suggested studies should be conducted into possible treatment via environmentally-friendly methods to minimise the release and impact of drugs from festivals.
They also suggested information on the harmful effects of public urination should continue to be provided to festival attendees.
Dr Christian Dunn, from Bangor University, explained: "Our main concern is the environmental impact.
"Education is essential for environmental issues, just as people have been made aware of the problems of plastic pollution, and Glastonbury have made great efforts to become plastic-free, we also need to raise awareness around drug and pharmaceutical waste - it is a hidden, worryingly-understudied yet potentially devastating pollutant."
A spokesperson for Glastonbury Festival said: "Protecting our local streams and wildlife is of paramount importance to us at Glastonbury Festival and we have a thorough and successful waterways sampling regime in place during each Festival, as agreed with the Environment Agency.
"There were no concerns raised by the Environment Agency following Glastonbury 2019.
"We are aware that the biggest threat to our waterways - and the wildlife for which they provide a habitat - comes from festival-goers urinating on the land.
"This is something we have worked hard to reduce in recent years through a number of campaigns, with measurable success.
"Peeing on the land is something we will continue to strongly discourage at future festivals. We also do not condone the use of illegal drugs at Glastonbury.
"We are keen to see full details of this new research, and would be very happy to work with the researchers to understand their results and recommendations."