More than 300,000 people have signed petitions delivered to Nestle in protest at its move to stop using Fairtrade cocoa and sugar in KitKats.
The chocolate wafer bar has been a flagship product of Fairtrade for a decade, but Nestle is cutting links with the non-profit organisation.
The Fairtrade Foundation said the move was "devastating" for thousands of cocoa farmers in the Ivory Coast.
Nestle said it will source KitKat cocoa on Rainforest Alliance terms.
The petitions were handed in at its UK KitKat factory in York, to coincide with the start of the annual cocoa harvest in the Ivory Coast.
The firm already uses Rainforest Alliance-certified farmers on other bars such as Aero and Yorkie.
The Fairtrade Foundation said vulnerable cocoa farmers would lose out on around £1.37m a year in premium payments, and would have less control over how they spend their money.
Sugar farmers will lose more than £500,000 in annual premium payments, it added.
Joanna Pollard, co-ordinator of Fairtrade Yorkshire, who started a petition which has gathered 284,000 signatures, said: "As soon as I heard the news that KitKats would no longer be Fairtrade, I knew this would be devastating for thousands of farmers.
"Under the Fairtrade system, they have a seat at the table and make their own decisions about where their money is spent. The farmers I've spoken to feel that selling their crop on Fairtrade terms is vital for their communities."
'Listened to farmers'
A further 20,000 people also signed a second petition started independently by the Co-operative Party.
"We want to help further improve the lives of cocoa farmers through a number of different initiatives and by increasing our overall investment in sustainable cocoa," a spokesperson said.
"Before we made this decision, and throughout our notice period with Fairtrade, we have listened carefully to farmers.
"We have used their input to create a comprehensive package to support their needs through the transition."