The Co-operative Group is being investigated by the Groceries Code Adjudicator after suspicions the retailer has broken supplier Code of Practice.
The Groceries Code Adjudicator (GCA) said it holds a "reasonable suspicion" that the retailer may have broken the Code through some of its practices.
These relate to de-listing, with no, or short, fixed notice periods.
It also relates to the introduction of benchmarking and depot quality control charges over a period from early 2016 to at least summer 2017.
Christine Tacon, the current GCA, said she took her decision after escalating her concerns with the Co-operative Group in line with her published collaborative approach to regulation.
She has decided that an investigation is necessary to fully understand the extent to which the Code may have been broken and the root causes of the issues as well as their impact on suppliers.
The GCA is now asking for more information from direct suppliers and others to determine whether the Code has been broken and if so, what further action to take.
'Root causes'
Ms Tacon has called for evidence to be submitted by 4pm on 3 May 2018.
Previous escalated concerns with the Co-op as part of a published collaborative approach were undertaken, but now an investigation is "necessary", Ms Tacon explained.
“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,” Ms Tacon said.
“It is now important that suppliers provide me with information to help my investigation. I am looking forward to hearing what they have to say about whether they have experienced any of the issues now being investigated and if so, the impact on them of the Co-op’s conduct.
She added: “All information I receive will be treated with complete confidentiality.”
