One of America’s most successful class action lawyers, William Lerach, is leaving his current law firm this week as prosecutors press forward with a probe of alleged kickbacks to investors who helped his former firm file securities lawsuits.

Mr. Lerach will resign from the San Diego-based Lerach Coughlin Stoia Geller Rudman & Robins on August 31, he announced in a written statement yesterday. He set up Lerach Coughlin after leaving the firm under scrutiny, Milberg Weiss, in 2004.

Mr. Lerach’s statement and a parting e-mail to colleagues, which was obtained by the Wall Street Journal, lacked the customary protestation of innocence and omitted the usual vow to fight any charges relating to the alleged kickbacks. Last month, a former Milberg Weiss partner, David Bershad, pleaded guilty in the case and implicated the firm’s leadership in the alleged scheme. Mr. Lerach told his colleagues that his departure “will end the investigation.”

He also alluded, without specifics, to “mistakes” he made and to an imminent resolution of the criminal case. No charges have been filed against Mr. Lerach, but a lawyer involved in the case said recently that Mr. Lerach had engaged in plea discussions with prosecutors.