Idemitsu Kosan's founding family, which is locked in a battle with the firm's management over a proposed buyout of a rival, said on Monday that it had increased its stake in the Japanese refiner to more than 28 percent.
The family bought more shares in Idemitsu after having its holding diluted when the company sold new stock earlier this year, Yohei Tsuruma, the family's lawyer, said in a statement.
The descendants of founder Sazo Idemitsu remain opposed to plans for a takeover of Showa Shell Sekiyu, Tsuruma added.
Idemitsu said in August that the new share sale had diluted the founding family's stake in Idemitsu to about 26 percent, from more than a third previously, eliminating the family's ability to veto the merger with Showa Shell.
In a rare public corporate battle, the founding family and the board have been disputing the merger for about two years, through court and shareholder actions.
The family, led by Shosuke Idemitsu, has said the two companies are too different for any merger to work, while the board says the combination is necessary to compete in Japan's shrinking oil products market.
A spokeswoman for the family declined to comment further when contacted by Reuters. (Reporting by Osamu Tsukimori, writing by Aaron Sheldrick; Editing by Himani Sarkar)