Farmers' leaders in Scotland have called for a trade tsar to ensure a fair deal between supermarkets and their suppliers.
Farmers are said to be too scared to complain about supermarkets
The National Farmers Union of Scotland (NFUS) said a code of practice was ineffective, as producers were afraid to complain about their treatment.
Suppliers were said to be frightened of reprisals from retail giants.
Farmers have said their investigations have shown some supermarkets were misusing their dominant position.
The Office of Fair Trading (OFT) introduced a code of practice for supermarkets three years ago, detailing guidelines about how retailers should treat suppliers.
A recent report by the OFT said there were virtually no breaches of the code.
But farming industry leaders have accused the big companies of charging for shelf space and repackaging, and also of unfairly changing terms of contracts.
The union has demanded that an independent regulator be set up to investigate anonymous complaints and protect suppliers.
The Scottish Green Party last year led a debate in the Scottish Parliament calling for a tougher code of practice for large supermarkets.
The party said that the leading supermarket chains were putting local farmers out of business.
However, the plan encountered opposition, with the Scottish Retail Consortium disputing the Greens' assertions.
The Greens said the code of practice governing supermarkets was not working.
Reiterating their support, party co-convener Shiona Baird said: "The farmers' union is absolutely right - we need a regulator to rebuild trust and fairness in the relationship between supplier and buyer.
"The market domination of the big four supermarkets has led to a monopoly over the supply chain and it is suppliers and communities who are paying the price.
"I hope that all the political parties will back the farmers and that we send a strong message to Westminster that an independent regulator is needed to support fair trade."
The party claimed the companies had a stranglehold on milk prices.
The following month more than 250 protesting dairy farmers stopped delivery trucks from getting into Asda's distribution depot in Falkirk.