Activist hedge funds are tightening their grip on the boardrooms of corporate America, according to the annual proxy season report of Institutional Shareholder Services, the world's biggest proxy voting agency.

ISS said hedge funds had wielded their rising influence by waging proxy contests, such as Nelson Peltz's successful bid for board representation at foodmaker Heinz. Over the 2006 season there were 21 such contests, up from 18 last year and 19 in 2004.