Seven & I will sell department store unit Sogo & Seibu on September 1, a source with direct knowledge of the matter said on Wednesday, triggering a plan for a walk-out at a flagship store in what would be Japan’s first major strike in decades.

Japan-based Seven & I, operator of the world’s largest convenience store chain, agreed last year to sell loss-making Sogo & Seibu to US fund Fortress Investment Group, but the deal has been delayed amid opposition from workers.

The union intends to carry out its threat for a strike at the flagship Seibu Ikebukuro store in Tokyo on Thursday, according to a union member.

Labour disputes are extremely rare in Japan, and strikes even more so. The last time workers walked out on a major department store was 61 years ago, and for less than a day, according to Japan’s biggest industrial union, UA Zensen.

A person with direct knowledge of Seven & I’s plan said its board would meet on Thursday to officially decide on the sale. The person asked not to be identified because the plan is not yet public.

Seven & I said nothing had been decided yet. Fortress said it had no immediate comment.

In its negotiations with management, Seibu & Sogo’s union had given Seven & I an August 29 deadline to drop its plan to sell the company on September 1, the union member said.

The union received no reply and would therefore carry out the strike on Thursday at the flagship store, where about 900 union members are employed, he said.

Sogo & Seibu said it would temporarily close the store on Thursday.

Fortress had initially planned to spend around US$1.71 billion on the acquisition but is likely to have lowered that price to about $1.5 billion, the source said.

• Reporting by Ritsuko Shimizu and Kantaro Komiya; Writing by Chang-Ran Kim; Editing by Simon Cameron-Moore, of Reuters.