Merchants have been frustrated for years by MasterCard  (MA - Get Report)  and Visa  (V - Get Report)  transaction fees that they claim are too costly. Now they're joined by U.K. consumers who may have paid higher prices because of MasterCard's charges and want some of their money back.

The shoppers are planning a class action lawsuit seeking £19 billion ($24.6 billion) in damages related to fees that they said had been "anticompetitive" and unlawfully high for 16 years. The U.S. credit-card processors are simultaneously facing legal claims from retailers such as Walmart (WMT - Get Report) and Home Depot (HD - Get Report)  . Last month, a federal appeals court overturned an antitrust settlement of $7.25 billion with the card companies, opening the door for potentially higher damages.

"The prices of everything we all bought from 1992 to 2008 were higher than they should have been as a result of the unlawful conduct of MasterCard," Walter Merricks, the U.K. consumers' representative, said in a statement. "This case should send a signal to companies that break competition laws at the expense of U.K. consumers that they do so at their financial peril."

Known as interchange fees, the charges at the center of the case are one component of swipe fees that represent the second- or third-highest operating cost for merchants, according to the National Retail Federation. Total swipe fees average about 2% per purchase. While consumers typically aren't aware of the fees, the costs can be passed on to them through higher retail prices.

The U.K. case will be the first filed under the country's Consumer Rights Act of 2015, which allows a group of people to sue for collective damages, and is the largest in the nation's history, according to lead litigation firm Quinn Emanuel Urquhart & Sullivan.

"MasterCard firmly disagrees with the basis of this legal claim," a spokesman for the Purchase, N.Y.-based company said in an email. "Electronic payments deliver real value to people online, in-store and everywhere."

The action creates little immediate risk to the company since it's not likely to begin before 2018, but there will be "some level of overhang," Keefe, Bruyette & Wodds analyst Sanjay Sakhrani wrote in a note to clients.

"While the total damages asked by the consumers is significant, we think it is too early to begin deriving potential losses to MasterCard," Sakhrani wrote, noting that the company generates about $4 billion cash a year and has ample ability to borrow.

The case's impact on customer loyalty and shareholder confidence, however, may be more severe, Boris Bronfentrinker, Quinn Emanuel lead partner, said in a statement.

While MasterCard rival Visa wasn't named in the U.K. lawsuit, because it followed European Commission caps on fees, the San Francisco-based company is facing litigation and merchant complaints elsewhere.

Last month, Walmart Canada said that Visa's credit card fees were too high and that it would no longer take the cards as payment, leading analysts to question whether Walmart would expand the ban into the U.S.

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Home improvement retailer Home Depot filed an antitrust case last month, saying that both Visa and MasterCard were involved in price-fixing and setting excessive fees for debit and credit card transactions. 

And further legal costs are likely from the 2012 antitrust case after an appeals court overturned the original $7.25 billion settlement because some of the merchants involved weren't properly represented.

"This 'settlement' was never a settlement on behalf of the retail industry, but rather a backroom deal that failed to represent the interests of retailers," Mallory Duncan, National Retail Federation senior vice president and general counsel, said in a statement. "It would have given merchants pennies on the dollar for the price-fixing they have suffered at the hands of the big credit card companies and would have done nothing to end price-fixing or to lower swipe fees going forward."

If the terms are renegotiated, Visa isn't directly liable for any of the settlement damages because the costs are covered through a litigation escrow account, funded by U.S. banks that hold Class B shares in the company, Sakhrani said. MasterCard may be liable for 12% of the settlement but "could be exposed to as much as 30% of the additional amounts," he added.

One piece of good news for both Visa and MasterCard: The U.S. Supreme Court agreed to hear both card processors' appeals in a class action case where consumers and independent ATM operators said the companies and other U.S. banks didn't allow the networks to charge a reduced ATM fee, which they considered "fee-fixing." 

"We're pleased that the Supreme Court has accepted our petition to hear our case and clarify where there may be a discrepancy between prior rulings," a MasterCard spokesman said in an email. 

Keefe, Bruyette & Woods rate both MasterCard and Visa as an "outperform" with price targets of $88.24 and $74.67, respectively. 

Visa stock has fallen 3.4% this year to $74.95, a better performance than MasterCard, whose stock dropped 9.4% to $88.16. Both lagged the broader S&P 500, which has gained 2.8%.