A federal judge in Manhattan denied a bid by Citigroup Inc. and Discover Bank to dismiss an antitrust suit accusing them of conspiring with American Express and Wells Fargo to impose mandatory arbitration clauses on credit card holders.

The ruling by Manhattan District Court Judge William Pauley III said bank officials hosted meetings in which the banks discussed their plans to incorporate mandatory arbitration provisions. Judge Pauley said an e-mail by Alan Kaplinsky of Ballard Spahr indicated that the banks may have collaborated to stave off class actions. The e-mail encouraged the bank officials to be "well networked" because "the plaintiffs' bar is engaged in a 'take no prisoners assault' on consumer arbitration programs."