Caixa has engaged lawyers and a bank syndicate to advise it on the best alternatives to divest the stake, according to the letter.

Reuters reported last week that Caixa's investment banking unit would lead the sale, along with Bank of America Corp , Morgan Stanley and UBS AG.

The Petrobras filing did not name the advisors.

According to the oil firm's website, Caixa owns 2.2 percent of its total capital stock, including 241,340,371 voting shares and 43,330,000 preferred shares.

Caixa did not rule out a follow-on share offering to investors in Brazil and in the United States, it told Petrobras in the letter.