The OFT is said to have found evidence of stores and suppliers sharing sensitive price information.
The consumer watchdog has written to more than 20 companies to say it has "reasonable grounds to suspect" data about pricing levels was shared through suppliers.
None of the companies has been accused of breaking the law, but if the investigation concludes that a cartel has been in operation, those companies involved face fines amounting to 10% of their annual turnover.
The investigation has been on going since April when the OFT launched an inquiry into potential breaches of competition law by a number of retailers and suppliers, across a range of product areas.
Dawn raids were carried out by the OFT earlier this year on a number of leading companies, kicking off the largest investigation of its kind by the OFT into a possible cartel.
Yesterday, the OFT said: "The OFT are conducting an ongoing investigation into potential breaches of competition by a number of retailers and suppliers across a range of product areas. The OFT have written to all the parties providing an update on the investigation."
The inquiry began after Asda uncovered information when going through internal email records last summer.
The supermarket's examination of its email trail was ordered by the Competition Commission, which was carrying out a separate investigation on the grocery sector.
In the wake of Asda's alert, the OFT became involved and launched dawn raids and demanded information from other companies.