NEW YORK—A federal judge last week ordered a unit of Aon Corp. and Marsh & McLennan Cos. Inc. to negotiate before moving forward on a lawsuit alleging Marsh stole Aon trade secrets, employees and clients—a charge MMC denies.

Aon Risk Services Northeast Inc., a New York unit of the Chicago-based broker, filed the suit last week in U.S. District Court in New York. The complaint names MMC, Marsh USA Inc. and three former employees of Aon's Trade Credit group—Michael Kornblau, Tyler Wendleken and Karryn Angoff—who left Aon in February to join Marsh USA, according to court documents.

Marsh and the employees allegedly stole business “through the patently illegal downloading of Aon proprietary trade secret information, unlawful disclosures of Aon trade secrets, and other unfair and unlawful activities,” according to the suit that alleged that MMC's goal was to obtain “a prepackaged book of business.”

The complaint describes Aon's trade credit insurance business as highly specialized and reliant on carefully cultivated relationships. It “can take months and sometimes years of working with a client before they decide to purchase this type of insurance. It's also true that those same clients tend to be repeat clients,” Aon said in its complaint, which seeks an injunction and damages.

Aon said it lost business after the former employees conspired with high-level Marsh executives to unlawfully solicit Aon clients to switch to Marsh before the Aon employees moved to Marsh, according to the suit. The dollar amount and clients that Aon alleges it lost are redacted in the suit.

In addition, Aon said the individuals it has sued promised in their employment agreements to protect from disclosure valuable information Aon considered trade secrets, including lists of clients and prospective clients, contract terms and conditions, copyrighted materials, business plans, compensation, revenues and other data.

However, Aon said, the individuals unlawfully hacked Aon's computer system to steal proprietary information before they left.

A computer forensic analysis conducted on the employees' laptops reveals a series of illegal downloads and deletions. For example, in the days prior to Mr. Kornblau's resignation in February, he attached five portable storage devices and accessed confidential client information including policy details and premium amounts, the complaint said.

Mr. Kornblau also downloaded an “Aon commission grid” that shows Aon's negotiated commissions for all lines of insurance globally—”highly competitive information,” according to the complaint.

At a meeting last week, U.S. District Court Judge Richard M. Berman in New York ordered the two parties to engage in good faith settlement negotiations, a source familiar with the matter said.

“The allegations in the complaint speak for themselves,” a spokesman for Aon said.

Reacting to the suit, a spokeswoman for MMC said: “We do not believe there is any merit to this litigation and will aggressively defend ourselves against it.”

Through an MMC spokesperson, the three individuals declined comment.

A hearing has not been set.