Lawyer emails threaten $6B settlement: Report

Visa and Mastercard settlement in jeopardy

A friendship between two opposing lawyers could shake up a $6 billion class-action settlement involving Visa and MasterCard.

Lawyers representing about 100 merchants are slated to tell the card operators that they want to scrap the deal reached in 2012 because of inappropriate communication between lawyers, The Wall Street Journal reported Monday, citing sources. The initial claims challenged rules and fees used by Visa and MasterCard, and the settlement permitted merchants to charge more to customers who pay with cards.

The agreement came into question during an investigation into alleged theft by Keila Ravelo, who represented MasterCard during the proceedings for law firm Willkie Farr & Gallagher. The Journal wrote that New Jersey authorities investigated Ravelo after charging her with conspiracy to commit wire fraud shortly after she resigned from the firm.

JB Reed | Bloomberg | Getty Images

Emails and documents sent between Ravelo and Gary Friedman, who represented merchants in the case, were then discovered, the report says. Ravelo and Friedman worked together at another firm earlier in their careers.

Read MoreVisa earnings sail past forecasts

Merchants plan to say that Ravelo and Friedman exchanged confidential information and documents, leading to inadequate representation, according to the report. Friedman represented American Express in another pending $79 million settlement, which could be threatened, as well, sources told the paper.

A MasterCard spokesman told CNBC "we believe recent events will not have any impact on the settlement and the outcome of the case will stand."

Ravelo's lawyer Steve Sadow said, "We are not in a position to comment at this time."

AmEx declined to comment. Visa and MasterCard did not immediately respond to CNBC's requests to comment. Friedman's firm and Ravelo's former firm also did not answer CNBC's requests.

Read MoreAmerican Express revenue comes in light