TOKYO (Reuters) -A labour union at Seven & i Holdings' department store unit Sogo & Seibu threatened on Monday to go on strike at a flagship store on Aug. 31 depending on the outcome of talks with management over the subsidiary's planned sale to a U.S. fund.

Labour strikes are extremely rare in Japan, and if it goes ahead, it would be the first at a major department store in about 60 years, local media reported. Sogo & Seibu's labour union announced the strike notice at a press conference.

Japan-based Seven & i, operator of the world's largest convenience store chain, agreed last year to sell Sogo & Seibu to Fortress Investment Group. But the deal has been delayed amid opposition from workers, and company management cancelled a meeting for Friday last week where it was to finalise the sale, Kyodo had reported.

Seven & i Holdings confirmed its unit had received the strike notice and said it was reviewing the union's requests. Negotiations will continue, it said.

If the talks break down, about 900 union members at Seibu's flagship store in Ikebukuro in Tokyo will strike on Thursday, a union member said. He said the union settled on that date because it believed Seven & i was looking to complete the sale of the unit on the following day, Sept. 1.

Sogo & Seibu's union had the support of the unions of rival department stores Takashimaya, Isetan Mitsukoshi, Daimaru Matsuzakaya and Hankyu Hanshin, all of which attended the press conference, the union member said.

(Reporting by Ritsuko Shimizu and Rocky SwiftWriting by Chang-Ran Kim Editing by Gerry Doyle and Mark Potter)