The European Commission has charged Visa with a breach of competition rules after the credit card scheme blocked an application for membership from US bank and rival card operator Morgan Stanley Dean Witter.
The ruling comes four years after MSDW first complained to the Commission about its enforced exclusion from the Visa network.
While MSDW - as the owner of the Discover network in the US - is refused Visa membership, the owners of several other payment card schemes, most notably Diners Club operator Citigroup, have been admitted as Visa members.
In its findings, the Commission says: "Visa has failed to explain why a global network such as Diners Club, which operates in over 200 countries, including Europe, should be less of a threat to Visa than a regional network like Discover, which operates only in the US and does not have any presence in Europe".
MasterCard, the Commission noted, has no such rule and has admitted MSDW to its scheme.
Visa has three months to respond to the Commission's objections and can request an oral hearing.