Britain's competition regulators said Tuesday they will make a detailed analysis of competition between grocers at the local level, and that it is seeking more information about how the big chains are treating farmers.
The Competition Commission, in an update on its inquiry, said it had not detected widespread problems in the relationship between supermarkets and suppliers.
"However, we have some concerns about farmers and we have not received as much specific evidence about unfair treatment of suppliers as we might have expected," the commission said.
Commission chairman Peter Freeman said the next focus would be at the local level.
"We are not here to punish success or individual retailers but we are concerned with whether Tesco, or any other supermarket, can get into such a strong position, either nationally or locally, that no other retailer can compete effectively," Freeman said.
Tesco is Britain's biggest supermarket chain, with more than a 30% market share. Asda, owned by Wal-Mart, is No. 2, followed by J. Sainsbury.
The commission added that it had not found a clear correlation between the size of buyers and better buying terms.