Dozens of people have been fined more than £2,000 in total for picking wild mushrooms in Epping Forest.
The City of London Corporation (CLC) said it handed out £80 fixed penalty notices to 27 people caught smuggling the fungi out of the ancient woodland over the last 12 months.
Some were caught carrying more than 5kg (11lb) of mushrooms, the CLC said.
Chairman of the CLC’s Epping Forest and Commons Committee, Graeme Doshi-Smith, said: “Fungi play an incredibly important role in the delicate balance of biodiversity which makes Epping Forest special.
“And many varieties are dangerous for human consumption and can indeed be fatal.”
Epping Forest Keepers say they also often issue verbal warnings to pickers.
Fungi are protected under Epping Forest bylaws and their large-scale removal damages its ecology, the CLC said.
The ancient woodland is designated as a Site of Special Scientific Interest and a Special Area of Conservation.
Many of its wild mushroom species are of national importance.
The CLC said certain types of fungi were sold to restaurants and markets.
Doing so removed food sources for rare insect species and other animals such as deer.
The fungi are important to the health of the site’s trees, some of which are up to 1,000 years old, as particular species protect their roots, and provide them with water and vital minerals.
The woodland has more than one million trees and around 500 rare and endangered insect species.