TOKYO, Dec 1 (Reuters) - Japanese gas company Iwatani Corp on Friday said it had raised its stake in Cosmo Energy Holdings to 20% by buying shares from funds related to prominent activist shareholder Yoshiaki Murakami for 105.3 billion yen ($710.8 million).

Iwatani already holds a 0.07% stake in Como. The price it paid for the 19.93% stake from the funds was at an 8% premium to Cosmo's 5,616 yen share price at Friday's closing. Iwatani plans to buy an additional 250,000 shares, or 0.29%, subject to regulatory approval, it said in a disclosure.

A Cosmo spokesman declined to comment on the deal.

Murakami's funds, which own 20% of Japan's third-biggest oil refiner, had planned to raise their stake to nearly 25%.

Cosmo called for a shareholder vote on Dec. 14, where it said it would seek approval for a revised "poison pill" strategy to block the additional fund purchases.

In a Friday statement issued after Iwatani's announcement, Murakami-linked fund City Index Eleventh said it has dropped the purchases plan for Cosmo shares.

Cosmo's chief executive Shigeru Yamada told a Reuters interview last month that the company could struggle to win shareholder support, as it decided to not use a tactic known as "a majority of minority vote", which it successfully used in a vote in June. ($1 = 148.1400 yen) (Reporting by Kantaro Komiya and Yuka Obayashi; Editing by Edmund Klamann and Miral Fahmy)