A serial plaintiff in securities lawsuits brought by a New York law firm, Milberg Weiss Bershad & Schulman LLP, has agreed to plead guilty to a conspiracy charge in connection with a federal investigation into allegations the firm paid millions of dollars in kickbacks to those who lent their names to court actions, according to court filings.

The guilty plea offered by Steven Cooperman, 64, of Fairfield, Conn., came as little surprise to those following the inquiry, since Cooperman, a former eye surgeon, has been cooperating with prosecutors for years. However, the court papers describing the charges against Cooperman did provide some new details about the government’s evidence against Milberg Weiss and its senior partners.

In May, Milberg Weiss and two of its partners, David Bershad and Steven Schulman, were indicted on charges that the firm defrauded the courts by making secret payments to Cooperman and others who served as plaintiffs. All pleaded not guilty and vowed to fight the charges at trial, which is set for next year.

The charges filed against Cooperman indicate that prosecutors have developed evidence against two of the top partners at Milberg in recent decades, Melvyn Weiss and William Lerach. Prosecutors allege that Mr. Weiss paid $175,000 to Cooperman in 1989 in the guise of a “phony art option” on a Picasso painting Cooperman owned, “Nude Before a Mirror.” In 1999, Cooperman was convicted of insurance fraud for staging the theft of that artwork and another, by Monet.

Mr. Weiss’s attorney, Benjamin Brafman, said the option was no sham, even though the class action lawyer never bought the so-called Reclining Nude. “Although Mr. Weiss changed his mind on that painting 20 years ago, he is now a leading collector of Picasso artwork,” Mr. Brafman said. “That was a completely legitimate transaction.”

Mr. Brafman also noted that no charges have been filed against his client. “This is old wine in a new bottle. This adds nothing to the mix,” he said.

Prosecutors did not make clear why they think the Picasso option was phony, but they may be relying in part on an assertion by Cooperman that the art deal was part of Milberg Weiss’s effort to funnel money to him.

An attorney for Milberg Weiss, William Taylor III, said Cooperman’s criminal background and plea deal undermine his claims. “They have never been credible and they are no more so today,” Mr. Taylor said. “This new plea bargain demonstrates Mr. Cooperman’s willingness to say anything for a shorter sentence and the government’s willingness to urge him to do so.”