Tesco revealed in December it was buying 77 c-stores from Mills Group through its One Stop business. The deal was subject to regulatory approval and the ACS called on the OFT to refer it to the Competition Commission.
However in a statement this week, the watchdog confirmed it had decided, on the information currently available to it, not to refer the deal.
"If Tesco sought to takeover a company operating 76 large supermarkets, then it would most likely be blocked. We fail to see any reason why this merger has been treated differently," said ACS chief executive James Lowman.
"A series of takeovers of c-store groups have taken place over the past eight years and none has been subject to a full competition investigation. In that time Tesco has grown from a c-store estate of about 80 stores to more than 1,700."
Lowman also criticised the OFT for not considering the ramifications of Tesco conducting the purchase using its One Stop business. "The OFT has therefore no answer to whether consumers are at risk of discrimination on price, quality, range and service," he added.