The Co-op is being investigated for how it treats its suppliers, with the supermarket regulator saying it has "reasonable suspicion" that the retailer broke its rules.
The Groceries Code Adjudicator, Christine Tacon, is looking into whether the retailer has been de-listing suppliers and charging them for quality control assessments.
The Co-op said it had already refunded £500,000 to more than 110 suppliers. The company is also writing to 1,500 suppliers to ask about de-listing decisions, and whether the processes around de-listing were fair.
Jon Whitfield, chief executive of Co-op Food, said: "We care deeply about our relationships with our suppliers and we are very sorry that in these two areas we have failed to live up to our usual high standards.
"We are already addressing the issues with the GCA and our suppliers and we hope the investigation will help bring to light any additional cases so that we can put these right as quickly as possible.”
Tacon said breaches of the code were made from early 2016 to summer 2017. She said suppliers must provide her with information to push forward the investigation, and that information provided will be treated as confidential.
“I have previously escalated my concerns with the Co-op as part of my published collaborative approach," Tacon said.
“However, after carefully considering all the information submitted to me, I have decided that an investigation is necessary so I can fully understand the extent to which the Code may have been broken and the root causes of the issues that have been raised with me."