Cosmo Energy Holdings will exclude its largest shareholder grouping from a vote on a "poison pill" takeover defense as the group's plans may conflict with the interest of general shareholders, its chief executive said on Wednesday.

The vote, to be held in June, would be Japan's second on a bid to exclude an investor from a shareholder vote on whether to adopt a poison pill, with implications for growing shareholder activism and future hostile bids in the country. Cosmo — 20% owned by the group led by prominent activist Yoshiaki Murakami — plans to implement a poison pill to dilute the group's stake if the group buys more shares without taking proper steps such as submitting a statement of purpose.

"A vote including the group may not properly reflect the opinion of general shareholders," CEO Shigeru Yamada said in an interview.