Cosmo Energy Holdings said on Tuesday it would call another shareholder vote in December to seek approval for a "poison pill" takeover defense against a group of activist investors, escalating the high-profile battle.

Japan's third-biggest oil refiner, however, said it will not prevent the group, led by prominent activist Yoshiaki Murakami, from voting this time, unlike the previous vote in June, which had excluded the group's 20% voting rights.

The tactics, a so-called "majority of minority" vote, has fueled concerns among governance experts, who argue that practice undermines shareholder equality and could embolden companies in Japan to hang on to much-criticized cross-shareholdings as a defense against hostile takeovers.