In April the EC hit Visa Europe with an antitrust charge, accusing the card network of violating competition rules over cross border interchange fees.
It emerged today that the commission has extended the deadline - to the end of July - for Visa Europe to respond to the charge.
EuroCommerce, which counts Tesco and Carrefour among its members, has now entered the fray and called on the EC to impose the same ruling on Visa as MasterCard faced in December 2007.
As a formal complainant the association will have full access to the case file and can add statements.
Xavier Durieu, secretary general, EuroCommerce, says: "The Visa interchange fee procedure is completely unfair. Retailers are forced to pay for a range of services from which they do not benefit. Bank rates are the only services which retailers, even the largest ones, are not able to negotiate."
A Visa Europe spokesperson says: "Eurocommerce's claims are unfounded and do not reflect the interests of consumers."
EuroCommerce first lodged a complaint against MasterCard and Visa 12 years ago as part of the tortuous interchange battle in Europe.
This contributed to Visa signing an antitrust agreement with the EU's Competition Commission in 2002, agreeing to reduce levels of interchange fees for processing card transactions in return for immunity from legal action.
However, following the expiration of the deal, last March the EC launched a probe into whether the interchange fees charged by Visa Europe "forbid restrictive business practices such as price fixing".
The firm moved to adopt a new methodology for setting cross-border fees, cutting the average from 0.7% to 0.61% but this did not satisfy the commission or EuroCommerce.
MasterCard capped fees at a much lower rate of 0.3% for credit cards and 0.2% for debit cards following an agreement with the EC as an interim measure in April pending a legal appeal against the Commission's ruling that the firm should scrap fees altogether.