A federal judge ordered retail trade groups Thursday to make changes to websites that seek to persuade merchants to oppose a proposed settlement over credit card swipe fees.

U.S. District Judge John Gleeson ruled that the websites present a biased view of the facts and, in turn, may have caused some retailers to oppose the settlement. During a hearing in Brooklyn, New York, Gleeson gave the parties a week to make changes to the sites.

The websites, which include Merchantsobject.com, were designed to make it easy for merchants to opt out of the multibillion-dollar legal settlement that certain retailers reached with Visa (NYSE:V), MasterCard (MA) and card-issuing banks.

The websites drew a rebuke from the merchants that negotiated the proposed settlement. That group, which includes the discount footwear chain Payless and a number of smaller retailers, asked Gleeson to require that changes be made to the sites.

Under the proposed settlement, merchants would receive about $6 billion from Visa, MasterCard and banks that issue their credit cards. But the card networks would continue to control how swipe fees are set, which has drawn howls from large segments of the merchant community.

Retailers that object to the settlement include 7-Eleven, Home Depot, Target, Walmart and a dozen retail trade associations. In trying to get the settlement rejected, they face an uphill fight because inaction by any individual retailer is tantamount to signaling its approval of the settlement deal.

Merchants face a May 28 deadline for retailers to register their objections to or opt of the proposed settlement. The judge will decide later whether to approve the settlement deal.