PALO ALTO, Calif. — One of America’s most successful class action and securities lawyers, William Lerach, is set to offer a guilty plea in connection with a long-running federal investigation into fraud involving his former law firm, Milberg Weiss, press reports and sources close to the case said.

Mr. Lerach is expected to offer a guilty plea to one felony conspiracy count, to agree to serve approximately one to two years in prison, and to forfeit a substantial amount, a person familiar with the agreement said. Under the terms of the deal, the famed trial lawyer will not be required to testify against other targets of the investigation, the source said.

The agreement is expected to be filed with Judge John Walter in Los Angeles today. If he does not accept the recommended prison sentence, Mr. Lerach would be free to withdraw the plea and seek a jury trial. Terms of the deal were first reported by the Web site of the Wall Street Journal.

Milberg Weiss and two of its name partners, David Bershad and Steven Schulman, were indicted in 2006 for aiding in a scheme to make undisclosed payments to investors who helped the firm file securities cases. Payments to the plaintiffs were often disguised as legal fees to paid to outside lawyers, prosecutors said. In July, Bershad pleaded guilty to a conspiracy charge and admitted in open court that the scheme operated in general terms as prosecutors asserted.

Last month, Mr. Lerach resigned from his latest firm, Lerach Coughlin Stoia Geller Rudman & Robbins. Sources said the plea deal guarantees no charges against that firm.