A federal judge on Tuesday ruled that Bill Essayli is unlawfully serving as Los Angeles' top federal prosecutor and ordered him disqualified from overseeing two criminal cases, rejecting the Justice Department's latest effort to preserve his authority.

Senior U.S. District Judge J. Michael Seabright, however, delayed the disqualifications from taking effect while he considers an anticipated government request for a stay pending appeal.

Although the order applies to two prosecutions, Seabright rejected the legal arrangement that has allowed Essayli to continue leading the office under the title of first assistant U.S. attorney. He concluded that the Justice Department had unlawfully delegated powers so sweeping that Essayli in effect occupied the U.S. attorney's position.

"In sum, Essayli is unlawfully serving as a 'de facto' United States Attorney," Seabright wrote.

After learning about Seabright's rul...