Almost one in five suppliers to the country's biggest supermarkets would not raise any issues with an independent adjudicator, mainly because they fear retribution, a new report has revealed.
A survey of 1,000 suppliers showed other reasons included not being confident that a complaint would remain confidential, or not being sure anything would be done.
The Groceries Code Adjudicator, Christine Tacon, released the findings at the second annual conference of the organisation, which was set up to oversee the relationship between supermarkets and their suppliers.
The adjudicator has launched one formal investigation, against Tesco, with the outcome expected within the next few months, and has arbitrated in two cases.
The number of suppliers reporting Grocery Code issues in their dealings with the 10 biggest retailers in the past year was 70%, down by 9% over the previous 12 months.
Issues included delay in payments, unjustified charges for consumer complaints and an obligation to contribute to marketing costs.
Ms Tacon said complaints about overcharging suppliers for packaging and design of products was one of the main issues she was dealing with.
She has "closed down" concerns that suppliers were being overcharged by retailers for handling consumer complaints about products they supplied.
Charges ranged from nothing to £45.
"In some retailers, up to 97% of complaints are resolved in-store, which is a cheaper way of handling matters.
"Of those retailers applying charges, some only do so in a small proportion of cases, but all retailers charge for serious failings such as product safety issues."
The survey also showed that 54% of those who raised issues with the adjudicator in the last year concerned Tesco, compared with 26% about Morrisons, 15% about Asda, 14% about the Co-op, 13% about Sainsbury's, 5% about Iceland, 1% about Marks & Spencer, Waitrose and Lidl, while none concerned Aldi.
The proportion of suppliers saying they would consider raising an issue with the adjudicator increased by 9% to 47% over the past year.
"We still have some way to go in important areas, but this is a clear sign we are on the right track," said Ms Tacon.
She added that it was "disturbing" that only 9% of small suppliers and 29% of total suppliers had received any training in the industry's code of practice.
"Retail buyers are well trained in the code, so for a supplier to challenge a request or requirement they need to know their rights."