SAO PAULO, Oct 18 (Reuters) - Shares in Brazilian retailers Lojas Americanas SA and Americanas SA were up on Monday after the companies announced that they are considering a full merger and listing on Brazilian stock exchange B3's Novo Mercado.

Lojas Americanas shares soared 17% to 6.23 reais in morning trading, while those of Americanas SA were up 2% at 38.21 reais. The broader Bovespa index fell 1.1%.

Analysts at Bradesco BBI said the potential reorganization would be positive for both retailers, as it would improve corporate governance by having just one class of shares listed on the Novo Mercado and simplify the firms' structure.

Americanas SA, formerly named B2W, has already received 6.3 billion reais ($1.15 billion) in assets spun off from Lojas Americanas in a deal announced earlier this year, but both Lojas Americanas and Americanas continued trading separately.

According to the firms, the merger and listing currently under discussion would happen prior to a previously planned listing of Lojas Americanas in the United States, which remains under consideration.

The companies said in a securities filing that they saw an "opportunity of a possible corporate reorganization prior to the international listing."

They said that an analysis of the listing on Novo Mercado is currently underway at an operational level, adding that "there are no decisions yet in this regard."

According to the filing, the move would require approval from the management and shareholders of both companies.

($1 = 5.4609 reais) (Additional reporting by Paula Arend Laier; Editing by Louise Heavens and Jan Harvey)