The US Supreme Court has upheld an earlier ruling allowing millions of retailers to join supermarket chain Wal-Mart in bringing an antitrust lawsuit against Visa and Mastercard.

The decision comes as a blow to the two credit card companies, which had asked the court to limit the number of litigants in order to prevent the likely damages bill from spiralling out of control.

The lawsuit, which traces its origins back to 1996, will now be settled in the Brooklyn court where it began.

"The case is now back on track," said the retailers' chief lawyer, Lloyd Constantine.

Debit pressure

The retailers allege that Visa and Mastercard used their dominance of the credit card market to force shops and supermarkets to accept their debit cards, which carry higher transaction costs.

The two companies account for more than three quarters of all US credit card payments.

Visa and Mastercard argued in court that allowing the expanded lawsuit to go ahead could leave them facing damages of up to $100bn, leaving them with little option but to settle.

But lawyers for the retailers said the credit card had deliberately exaggerated the likely damages bill in order to maximise its "shock value."

Credit card firms fight on

The retailers estimate preliminary damages at $8bn.

The credit card companies stressed that the Supreme Court decision has no bearing on the final outcome.

"As this case moves forward, we look forward to addressing the merits and other important matters with the court," said Visa's vice-president Kelly Presta.