(Adds information about ValueAct relationship with Disney in paragraph 3)

March 11 (Reuters) - Blackwells Capital said on Monday Walt Disney failed to tell shareholders that ValueAct Capital Management had invested more than $350 million of the U.S. entertainment giant's pension fund assets.

"ValueAct's management of Disney's pension funds is not disclosed anywhere in any of the referenced communications," Blackwells Chief Investment Officer Jason Aintabi wrote in a public letter to Disney shareholders.

Disney had fully withdrawn its investment in ValueAct and the firm no longer managed any money for the company when it built its stake in Disney last year, a person familiar with ValueAct's business said.

A ValueAct representative declined to confirm any details about the firm's current or past clients.

ValueAct, which began investing in Disney late last year and reached an information sharing agreement in January to help advise Disney on strategic matters, last week publicly supported the Disney board at the Council of Institutional Investors conference.

Disney is relying on ValueAct's endorsement at a time it is fighting to fend off two other activist-investors - Blackwells and Trian Fund Management - as each firm lobbies for seats on the Disney board.

Disney did not immediately respond to requests for comment.

Like many hedge funds, ValueAct and Trian earn fees by investing capital on behalf of big corporations.

Disney on Monday said it had fired Trian in 2021 for poor performance. But the company did not say in its proxy materials which other firms have previously or are still managing money for its pension fund.

Blackwells cited Disney's form 5500, which is filed with the Department of Labor, as its source for information on the management firms Disney employs. (Reporting by Chandni Shah in Bengaluru and Svea Herbst-Bayliss; Editing by Sherry Jacob-Phillips and Lincoln Feast.)