One of Yahoo’s (NASDAQ: YHOO) major investors are serving notice Marissa Mayer and those working alongside her. Making good on its threats, hedge fund Starboard Value called for the removal of Yahoo’s entire nine-member board on Thursday. The move is a significant escalation in an 18-month-long battle that could lead to Mayer’s ouster as CEO or the eventual sale of Yahoo. Starboard said it will nominate an alternate slate of directors to the board at Yahoo’s shareholder meeting set for later this spring. Starboard told Yahoo earlier in the year that it would take that action if Yahoo didn’t take more aggressive steps to boost the company’s stock price. Starboard said it believes the current board “lacks the leadership, objectivity, and perspective needed to make decisions that are in the best interests of shareholders.” The hedge fund’s letter to Yahoo was first reported by The Wall Street Journal. Yahoo’s board issued a statement, saying it is reviewing Starboard’s proposal, and it will respond “in due course.” Starboard owns nearly 2% of Yahoo. Its stake is worth $570 million U.S. In an effort to stave off Starboard’s aggressive tactics, Yahoo in February laid the groundwork for a revitalization. The company laid off 15% of its workforce, eliminated products that weren’t working, returned focus to its core products and media brands and promised to save $400 million U.S. It also promised to sell off assets to bring in an additional $1 billion U.S. Yahoo’s stock closed trading Thursday — and a short week — at $34.75 U.S., at about the midpoint of a 52-week trading range from $26.15 to $46.17 U.S.