TOKYO (Reuters) - Japan's Fast Retailing Co Ltd (9983.T: Quote, Profile, Research) said on Tuesday it may merge three struggling subsidiaries as part of the apparel group's restructuring efforts.

Fast Retailing, whose Uniqlo casual clothing brand has often been called the Gap Inc (GPS.N: Quote, Profile, Research) of Japan, has expanded its operations through aggressive acquisitions, with an aim to double its annual sales to 1 trillion yen ($10 billion) by 2010.

While its flagship Uniqlo business has been on a recovery path after revamping its merchandise policy, the lacklustre performance of some group units has been a drag on the company.

Fast Retailing said it was considering merging casual clothing chain operator G.U. Co Ltd and shoe retailers Onezone Corp and Viewcompany Co Ltd 3033.Q.

"We have determined it is difficult for each firm alone to improve its sluggish performance and we need to make more drastic restructuring efforts," Fast Retailing said in a statement.

The three units posted losses in the most recent business year, the firm said.

Viewcompany, listed on the Jasdaq market for startups, is set to become a fully owned subsidiary after Fast Retailing acquired a 99 percent stake via a tender offer earlier this year.

Before the announcement, shares of Fast Retailing closed up 2.1 percent at 8,970 yen, outperforming the benchmark Nikkei average's .N225 1 percent gain.

(Reporting by Taiga Uranaka; Editing by Chris Gallagher)