The two firms already face a litany of lawsuits in London over so-called multilateral interchange fees, which retailers pay when consumers use a card to shop.

Visa and Mastercard are each being sued by hundreds of claimants at London's Competition Appeal Tribunal, which is currently managing the various cases together.

Special purpose vehicle Commercial and Interregional Card Claims (CICC) brought another set of lawsuits against Visa and Mastercard last year, seeking damages on behalf of merchants which were allegedly overcharged.

CICC is asking the tribunal to certify its cases under the United Kingdom's collective proceedings regime, which is roughly equivalent to the class action regime in the United States.

Its lawyer Michael Bowsher said on Monday that claimants in its cases would include small businesses who had not sued Visa or Mastercard because of a lack of resources or their "risk appetite".

He argued that permitting CICC's lawsuits to proceed would "ensure that the benefits of justice and compensation are not limited to a narrow slice" of potential claimants.

Bowsher said in written arguments that the majority of potential claimants – as many as 85% in the case of Visa – have not yet brought lawsuits.

But Visa and Mastercard's lawyers say the need for collective proceedings is undermined by the fact that thousands of merchants have already sued them.

Visa's lawyer Brian Kennelly said in court filings that more than 2,300 UK claimants are currently suing Visa, with more than 250 having a turnover of less than 5 million pounds ($4 million).

"These proposed collective proceedings are not, therefore, about providing access to justice to those who cannot seek it for themselves," he added.

Mastercard's lawyer Sonia Tolaney also said the cases should not be certified, arguing in court filings that CICC's cases could disrupt existing lawsuits.