Petrobras, as the company is commonly known, gave no reason for the resignation, in a note announcing the resignation sent to Brazil's securities regulator CVM.

Monforte was not immediately available for comment after normal Brazilian business hours on Friday.

The board on Wednesday voted 7 to 3 to approve audited financial statements that have been delayed since November after a corruption scandal raised doubts about the true value of Petrobras assets.

In the results, Petrobras announced a 21.6 billion real (4.82 billion pound) 2014 loss, the largest in the company's history, and wrote down about $17 billion of assets. Of the write-down, about $2 billion was the result of corruption.

Monforte is one of three non-government members on the Petrobras board. The two other members voted against approval of the accounts, Petrobras said in a separate statement on Friday.

Had he not resigned, Monforte, who represents non-government holders of the company's preferred stock, would have seen his position on the board end Wednesday, April 29 when minority shareholders elect a new representative to the board.

Monforte had not been nominated to serve another term.

(Reporting by Jeb Blount; Editing by David Gregorio)