U.S. Securities and Exchange Commission Chairman Mary Schapiro said her chief regret in three years running the agency was the failure to beat a court challenge of a rule letting shareholders put their own board candidates on corporate ballots.

"I would love to have found a way to structure that rule and its cost-benefit analysis in such a way that it could have withstood the court challenge that was brought against it," Schapiro said today in an interview with former SEC Chairman Arthur Levitt to air on Bloomberg Radio. "I still believe that it could be an incredibly important tool for shareholders to hold boards accountable for the conduct of companies."

The so-called proxy access rule, which cost the agency $2.5 million to write and defend, was rejected in a July 22 ruling by the U.S. Court of Appeals in Washington. The court cited an SEC failure to fully study the rule's cost to companies — inspiring a "redoubling" of agency efforts to study cost-benefit effects of its new rules, Meredith Cross, chief of the SEC's Division of Corporation Finance, said in November.