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Ms. Solis, a former congresswoman from California, told colleagues in an e-mail that she had submitted her resignation letter to Mr. Obama Wednesday afternoon.

She said she had decided to step down after consulting family members and friends. Associates of Ms. Solis, who is 55 and was born in Los Angeles, said she was likely to run for a seat on the Los Angeles County Board of Supervisors.

In a statement, Mr. Obama said, "Secretary Solis has been a critical member of my economic team as we have worked to recover from the worst economic downturn since the Great Depression and strengthen the economy for the middle class."

Ms. Solis was praised by labor unions for working to enforce workplace regulations and occasionally criticized for not being responsive to business interests. Among her biggest campaigns was cracking down on farmers who employ children or underpaid workers.

Ms. Solis, who was never viewed as part of Mr. Obama's inner core of advisers, prided herself on going to bat for the nation's recession-battered workers, especially those on the lower end. Many Hispanic workers looked to her as their champion – she focused, for instance, on the high fatality rate for Hispanic workers.

She made the department more aggressive in ferreting out rules violations involving the minimum wage, overtime and other issues. Last year the Labor Department collected more back pay for wage violations than in any previous year, more than $280 million on behalf of 300,000 workers.

Under her, the Labor Department incurred the coal industry's wrath by focusing on mine safety violations after the Upper Big Branch disaster, a West Virginia coal mine explosion in 2010 that killed 29 workers.

She is one of two Hispanic members of the cabinet, with Ken Salazar, the interior secretary, and was one of a handful of women, along with Secretary of State Hillary Rodham Clinton; Lisa P. Jackson, the administrator of the Environmental Protection Agency; Janet Napolitano, the secretary of homeland security; and Susan E. Rice, the ambassador to the United Nations, who has cabinet status.

Ms. Jackson has announced her resignation, and Mrs. Clinton is to be succeeded by Senator John Kerry..