Foragers are damaging the environment by stripping ancient woodland of wild mushrooms, officials have warned.
Commercial pickers are targeting Epping Forest, on the border of London and Essex, damaging the trees that rely on fungus to protect their roots.
Their ruthless and large-scale foraging is also depriving insects and animals such as deer of a valuable food source, the City of London Corporation said.
The corporation, which owns and manages the forest, warned the fungus pickers that they could be fined or prosecuted for gathering mushrooms.
The mushrooms are protected under Epping Forest byelaws.
Fungi play a key role in the forest’s wildlife habitats and are a major reason it has protected status including as a Site of Special Scientific Interest and a Special Area of Conservation, the corporation said.
Since 2014, 18 people have been prosecuted for foraging, while forest keepers often issue verbal warnings.
In the most recent incident, legal action is being pursued over the seizure of a haul of mushrooms weighing 108lb (49kg).
Graeme Doshi-Smith, chairman of the City of London Corporation's Epping Forest and Commons Committee, said: "Fungi play an incredibly important role in the delicate balance of biodiversity which makes Epping Forest special.
"We welcome the millions of people who come to enjoy this protected site. But I urge our visitors to leave the fungi how they find them - untouched.
"Our job is to protect this ancient woodland for everyone to enjoy. Hoovering up fungi on such a large scale is ecologically damaging and is simply unsustainable."
Epping Forest, London and Essex's largest green space, has more than a million trees, including ancient pollards of beech, hornbeam and oak.
It is home to around 500 rare and endangered insect species.
