(Updates with response from a Visa spokesperson in the sixth paragraph.)

European Union antitrust authorities are deepening their probe into Visa (V) and Mastercard (MA), sending detailed questionnaires to retailers and payment firms to assess concerns about opaque fee structures, Reuters reported Wednesday, citing documents.

Regulators are exploring whether clearer, standardized fee summaries -- categorized by type and written in plain language -- could help address longstanding complaints from merchants and payment providers, the report said.

The EU is also examining whether fines issued by the card networks should be invoiced separately, making it easier for businesses to understand and challenge them, the report said.

The latest round of 11-page questionnaires, sent out Tuesday, suggests that the investigation has progressed, with responses due by June 18, the report added.

"Mastercard offers consumers and businesses choice, ways to pay and be paid that are hassle-free and worry-free, secure and most convenient for them," the company said in a statement e-mailed to MT Newswires.

Visa's fees reflect the value it provides to financial institutions, merchants and consumers in Europe, including security and fraud prevention, operational resilience and reliability, consumer protections and "high-quality, innovative" products and services, a Visa spokesperson said in a statement e-mailed to MT Newswires Thursday.

(Market Chatter news is derived from conversations with market professionals globally. This information is believed to be from reliable sources but may include rumor and speculation. Accuracy is not guaranteed.)