WASHINGTON — A subsidiary of insurance giant Prudential Financial Inc. agreed yesterday to pay $600 million in fines and restitution to avoid being prosecuted on charges that it helped favored customers make improper mutual fund trades.

The settlement is one of the last and biggest to stem from the 2003 mutual fund scandal uncovered by New York Attorney General Eliot Spitzer and the first to include federal criminal charges.

Spitzer's revelations that mutual fund firms had cut secret deals with big investors known as hedge funds shook the $9.3 trillion industry to its core and sparked a host of investigations. Over the past three years, the Securities and Exchange Commission has brought more than 90 civil cases involving market timing and late-trading abuses and has collected more than $3 billion that will be returned to investors, SEC enforcement director Linda Chatman Thomsen said at a news conference announcing the Prudential settlement.

Prudential's brokerage subsidiary, now known as Prudential Equity Group LLC, admitted criminal wrongdoing, but prosecutors agreed to drop the charges if the firm stays out of trouble for five years. The company said it has set aside sufficient reserves to cover the $600 million payment, but the size is significant. Prudential's financial services businesses reported a second-quarter profit of $424 million.

Three former employees of Prudential's Boston office also pleaded guilty to federal criminal charges of inventing identities and fake accounts to help customers circumvent rules against market timing — industry slang for rapid in-and-out investments in mutual funds.

Prudential's management received more than 1,000 letters and e-mails complaining about the improper trades but failed to address the problem or discipline the brokers involved, Thomsen said.

"This is a major victory," particularly for small investors, Deputy Attorney General Paul McNulty told a news conference. "Mutual fund shares now represent a large portion of the life savings of the average American."

Of the $600 million, $270 million will go to a fund administered by the Securities and Exchange Commission to compensate the victims.

Market timers exploit the fact that mutual funds are priced once a day. They buy a fund when they think the constantly fluctuating value of its underlying stocks is greater than the once-a-day price reflects and then sell it a few days later. Many fund companies forbid the practice because the rapid turnover cuts into long-term returns by forcing managers to keep more of their funds' assets in cash and also because the rapid turnover can make it hard to stay focused on an investment strategy.

The payment is the largest by a single company to resolve market timing charges, McNulty said. In 2004, Bank of America Corp. agreed to pay $675 million, including fee cuts, to settle allegations of illegal after-hours trades and improper market timing, but that settlement also covered allegations against FleetBoston Financial Corp., with which it had merged.

McNulty said the Prudential scheme was particularly egregious because mutual fund investments, the main way Americans save for retirement and education, "have become a necessity" and "represent a large portion of the life savings of the average American."

Under terms of the deal, Prudential Financial will be required to make periodic reports about its compliance with securities regulation to its audit committee and to Michael Sullivan, the U.S. attorney in Boston, who said yesterday that the deceptive practices "undermined the integrity and utility of the automated, standardized mutual fund trading system."

Of the $600 million payment, $300 million will go to the U.S. Treasury; $270 million will go into a fund controlled by the SEC and ultimately will be returned to investors; $25 million will go into a Postal Inspection Service consumer fraud fund; and $5 million will go to Massachusetts, which took a lead role in bringing the case against Prudential in 2003.