By Makiko Yamazaki

TOKYO (Reuters) -Shareholders of Cosmo Energy Holdings approved a "poison pill" takeover defence on Thursday, in a controversial vote that excluded certain activist shareholders and has implications for hostile takeover attempts in Japan.

It marks the second such vote on a poison pill in Japan that excludes an investor, a so-called majority-of-minority vote, a practice some governance experts say could serve as a new weapon against shareholder activism.

Cosmo, Japan's third-biggest oil refiner and 20% owned by a group led by prominent activist Yoshiaki Murakami, can now implement a poison pill to dilute the activists' stake if the group buys more shares without taking steps such as stating its purpose.

The company reiterated at Thursday's general meeting that the exclusion of the group from the vote should be justified to "reflect the will of general shareholders", who it believes could be pressured into selling their shareholdings because effective control by the group "would damage corporate value".

The group, which called for a spin-off of Cosmo's renewable energy business and a consolidation of refineries, has argued that such a "forcible and oppressive" voting method would only raise the risks of management entrenchment and potentially set back corporate governance in Japan.

Majority-of-minority conditions are typically applied to protect general shareholders in a buyout by a controlling shareholder, but have never been used outside Japan to prevent certain shareholders from voting.

Governance experts point to concerns that such votes would fly in the face of shareholder equality and embolden companies in Japan to hang on to much-criticised cross-shareholdings as a defence against hostile takeovers.

"The notion that elected directors can on their own authority pick and choose which shareholders may vote stands shareholder democracy on its head," Stephen Givens, a corporate lawyer based in Tokyo, said.

Drafts of mergers and acquisitions (M&A) guidelines that the industry ministry is compiling state that majority of minority votes "may be permitted only in very exceptional and limited cases", noting that such votes could discourage even desirable takeovers.

(Reporting by Makiko Yamazaki; Editing by David Dolan and Muralikumar Anantharaman)