In a letter to Unizo's board, U.S. activist fund Elliott Management ended weeks of public silence over Unizo's abrupt withdrawal of support last month for a $1.3 billion buyout bid. That about-face came after Unizo had earlier welcomed the "white knight" bid from SoftBank-backed Fortress Investment Group.

Previously a relatively obscure hotelier, Unizo is now being seen by foreign investors as a prominent battleground for corporate governance and whether Prime Minister Shinzo Abe's push for greater transparency and reform has taken root.

"We are highly concerned about the lack of disclosure and the risk of conflicts of interest that have appeared in Unizo's handling to date of the tender offer bids," Elliott, which owns 13.14% of Unizo, said. It was referring to both the Fortress offer, and an earlier, hostile bid from H.I.S. Co, which has since been withdrawn.

"No answers, or answers that do not directly address shareholders' concerns, will only deepen the misgivings that shareholders are starting to have," Elliott said.

(Reporting by Junko Fujita and David Dolan; Editing by Chris Gallagher)