Mexico's competition authority is reported to be investigating an alliance formed by three leading Mexican retailers, aimed at improving competitiveness against Wal-Mart de Mexico (Walmex).

Grupo Gigante, Soriana and Controladora Commercial Mexicana, last year formed Sinergia, a joint purchasing venture aimed at cutting costs and getting better prices from suppliers, in a bid to fend off intense competition from Walmex, which is majority owned by US retail titan Wal-Mart.

An official from Mexico's Federal Competition Commission, or CFC, said the agency was requesting further information about the deal, reported Reuters.

"We are concerned that the operation of Sinergia could violate the law," Reuters quoted its agency source as saying.