Yahoo said it “noted” Starboard's announcement regarding the board nominations. The board's nominating and governance committee will review the nominees and respond “in due course,” Yahoo said in a statement.

For Mayer, who sits on the board, it's another difficult step in her tenure that began as a turnaround effort when she arrived in July 2012. In February, she rolled out her latest plan to overhaul the company, calling for employee cuts, product closures, and other reductions to help drive efficiency and focus on growth opportunities.

Earlier this month, Mayer said she would do what's best for investors as Yahoo considers its strategic options and she would like to keep her job leading the company even if it changes hands.

“I certainly hope the strategic alternative has a place for me,” she said in an interview on the “Charlie Rose Show” television program. “But that said, we'll obviously honor our commitments to our shareholders.”

Mayer stands to collect at least $12.4 million if she's terminated, including $3 million cash severance and about $9.34 million in equity vesting early and certain health and outplacement benefits, according to data compiled by Bloomberg.

Yahoo shares fell 1.8 percent to $34.19 at 9:46 a.m. They had gained 4.6 percent this year through Wednesday.

Nomination of Entirely New Board 'a Little Surprising'

“The fact that Starboard chose to nominate a full slate of directors was a little bit surprising,” said Paul Sweeney, an analyst at Bloomberg Intelligence. “It shows that they are not comfortable with the pace of restructuring that the management team and the board have undertaken so far.”

Earlier this month Yahoo added two new independent directors who have experience helping sell companies as it sought to bulk up the board to prepare for a proxy fight.

Starboard met with Yahoo on March 10, the day Yahoo appointed Eric Brandt and Catherine Friedman to the board, according to a person familiar with the matter. Starboard, who was not alerted about the new appointments prior to the meeting, has had contact with the Web portal since then, said the person, who asked not to be identified because the matter is private.

Starboard's outreach to Yahoo shareholders through proxy advisers has indicated tremendous support for its efforts, which gives it confidence in initiating the proxy fight, according to the same person.

Starboard is one of the most prolific U.S. activist investors and has a track record of successfully pushing companies to heed its wishes. In 2014, Starboard persuaded investors to replace Darden Restaurants Inc.'s entire 12-member board after the unpopular sale of its Red Lobster chain to Golden Gate Capital. Starboard also recently pressured office-supply rivals Staples Inc. and Office Depot Inc. into a merger.

Starboard held less than 1 percent of Yahoo stock at the end of 2015, according to data compiled by Bloomberg.

The fight with Starboard isn't Yahoo's first run-in with disgruntled activists. In 2012, Third Point LLC's Dan Loeb succeeded in getting himself and two nominees on the Yahoo board after tangling with former CEO Scott Thompson, who stepped down after failing to correct errors in his credentials. Later that year, Loeb was instrumental in getting Mayer to lead the company.

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