A woodland is being stripped of wild mushrooms by pickers selling them off to restaurants and markets, it is claimed.
The City of London Corporation (CLC) said commercial foragers had been targeting London's Epping Forest.
It said the removal of mushrooms threatened its biodiversity and deprived animals such as deer of a food source.
Ancient trees also rely on fungus to protect their roots, it added.
The corporation, which owns and manages the forest, is warning the fungus pickers that they could be fined or prosecuted for gathering mushrooms, which are protected under Epping Forest byelaws.
Since 2014, 18 people have been prosecuted for foraging, and forest keepers often issue verbal warnings.
In the most recent incident, involving legal action, a haul of mushrooms weighing 108lb (49kg) was seized.
Fungi play a key role in Epping Forest's wildlife habitats and are a major reason it has protected status.
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.
"Hoovering up fungi on such a large scale is ecologically damaging and is simply unsustainable."
According to the CLC, Epping Forest, London and Essex's largest green space, has more than a million trees, including ancient pollards of beech, hornbeam and oak, and is home to around 500 rare and endangered insect species.
The Forest covers around 2,500 hectares and stretches from Manor Park to just north of Epping.
Two thirds of the Forest has been designated a Site of Special Scientific Interest (SSSI) and a Special Area of Conservation (SAC).