Brazil's largest bank, Itau-Unibanco, will exercise its option to acquire 24 million shares of local credit-card networking services provider Redecard SA (RDCD3.BR) through a secondary offer on the Brazilian Stock Exchange, or Bovespa, Redecard said late Monday in a statement.

Earlier this month, Citigroup Inc. (C) announced its plan to sell at least 82 million of its shares in Redecard. Citigroup also said it could increase the offer through an extra lot of shares, selling a total of 106.7 million shares.

The price of the shares under the offer will be set after the conclusion of the bookbuilding period, the company said.

Redecard is controlled jointly by Brazilian banking giant Itau-Unibanco and Citigroup. Currently, Itau-Unibanco has a combined 46.42% stake in Redecard, while Citigroup has a 17% stake.

Itau-Unibanco was formed late last year by the merger of Itau (ITU), at that time Brazil's third-largest bank, and Unibanco (UBB), at that time the country's fourth-largest bank. The new merged bank is Brazil's largest financial institution. Itau and Unibanco shares still trade separately but will be unified into a single new share March 31.

Citigroup, through its Citibank division, has total of 114.4 million Redecard's shares, according to the prospectus of the offer. Citigroup is the lead coordinator of the operation.

The shares under the offer are set to begin trading Thursday.

-By Rogerio Jelmayer, Dow Jones Newswires; 5511-2847-4521; rogerio.jelmayer@dowjones.com