SAO PAULO, May 6 (Reuters) - Brazilian electricity company Cemig said on Thursday it is opening up a competitive tender for its 21.7% stake in power transmission firm Transmissora Aliança de Energia Elétrica (Taesa).

Cemig said in a securities filing that 218,370,005 common shares and 5,646,184 preferred shares in Taesa will be sold via a competitive auction, with the auction result scheduled to be ratified by Aug. 16 this year.

Chief Executive Officer Reynaldo Passanezi had said at the end of March that Cemig aimed to sell its stake in Taesa by the end of this year.

Cemig controls Taesa with its partner ISA Brasil, a unit of Colombia's ISA, which has a 14.9% stake. (Reporting by Jamie McGeever Editing by Chris Reese)