Jon Corzine has resigned as MF Global Holdings Ltd's chairman and chief executive officer four days after the futures brokerage filed for bankruptcy protection, culminating a rapid downfall for one of Wall Street's best-known executives.

Corzine said his decision was voluntary and was best for the company and its stakeholders.

I feel great sadness for what has transpired at MF Global and the impact it has had on the firm's clients, employees and many others, Corzine said. I intend to continue to assist the company and its board in their efforts to respond to regulatory inquiries and issues related to the disposition of the firm's assets.

Friday's resignation is the latest development in a stunning downfall for Corzine, 64, who ran Goldman Sachs & Co in the late 1990s and was subsequently a U.S. senator from New Jersey and governor of that state. MF Global, which Corzine joined in March 2010, had been his ticket back to Wall Street.

Corzine is not seeking severance, the company said. He had been entitled to a $9 million payout if he were let go without cause, a July regulatory filing shows.

The bankruptcy filing came after MF Global's bets on European sovereign debt scared away clients, counterparties and investors. It was accelerated after major credit rating agencies downgraded the company to junk status last week.

U.S. regulators, meanwhile, are conducting a broad review of the company's business as they try to track down more than $600 million of missing customer money.

In a statement, MF Global said Bradley Abelow, its chief operating officer, and Edward Goldberg, its lead director, will remain in their positions.

Brokerages such as MF Global are required to keep customer money segregated from their own cash. Questions about whether this took place at MF Global have attracted the attention of the Federal Bureau of Investigation as well as regulators.

Neither MF Global nor Corzine has been accused of wrongdoing.

Corzine has hired leading white-collar defense lawyer Andrew Levander of Dechert LLP to represent him in cases that might stem from the bankruptcy filing, a legal source briefed on the matter said on Thursday.

Levander has represented outside directors of Lehman Brothers Holdings Inc and former Merrill Lynch & Co chief John Thain, among others. Corzine has hired separate counsel for the bankruptcy case.

We will look at every aspect of how the firm conducted business, Mary Schapiro, chairman of the U.S. Securities and Exchange Commission, told Reuters on Thursday.

(Reporting by Jed Horowitz, Jennifer Merritt and Jonathan Stempel in New York; Editing by Derek Caney and Lisa Von Ahn)