BRUSSELS (Reuters) -EU antitrust regulators are asking retailers and payments companies whether a standardized summary of fees by Visa and Mastercard ( MA ) and more transparency on the charges would address their concerns, according to documents seen by Reuters.

The latest questionnaires sent on Tuesday, which came nearly two months after the last batch, suggest that EU regulators have escalated their preliminary investigation into Visa and Mastercard ( MA ).

The two companies, which charge scheme fees for services related to participation in their card system and process about two-thirds of card payments in the euro zone, have long faced complaints from merchants and payments companies.

The new questionnaires focus primarily on a simpler and more transparent fee structure and how Visa and Mastercard ( MA ) should handle fines levied on retailers and payments companies.

Visa and Mastercard ( MA ) did not immediately respond to requests for comment.

Respondents were asked whether a standardized summary of fees categorized by type by Mastercard ( MA ) and Visa, including descriptions and services covered, and drafted in plain and intelligible language would be a suitable solution.

The 11-page questionnaires asked if changes to contracts including terms and conditions, services and fees should be based on objective, transparent, general and non-discriminatory criteria.

Regulators wanted to know if a requirement for card schemes to invoice fines separately would help retailers and payments companies to identify the reason for the levies and to challenge them when appropriate as well as the process for contesting them.

The deadline for replies is June 18.