Barnes & Noble Co. said activist investor Ronald Burkle was unqualified to be on its board and accused the billionaire of trying to take over the bookseller through a proxy battle without rewarding shareholders.

“We believe Los Angeles-based investor Ronald Burkle is trying to take control of Barnes & Noble without paying you the premium you deserve for your shares,” the company’s board of directors wrote in a letter to shareholders Wednesday, calling Burkle’s efforts a “scheme.”

Burkle is running an alternate slate of three directors, including himself, for election to Barnes & Noble’s board at the company’s annual meeting on Sept. 28. Burkle holds 18.8 percent of Barnes & Noble’s shares, making him its second-largest shareholder.

Leonard Riggio, the bookstore chain’s chairman and largest shareholder with a stake of 28.2 percent, is up for re-election and the company has nominated two outside executives alongside him.

Earlier this month, Barnes & Noble put itself up for sale. But the subsequent proxy battle with Burkle is seen as complicating or even derailing the sale process.

Barnes & Noble also asked shareholders to reject Burkle’s proposal to increase the threshold of its anti-takeover “poison pill” to 30 percent from 20 percent. Barnes & Noble put the pill in place in November in response to Burkle’s quick accumulation of shares.

In its letter to encourage shareholders to support the company-nominated directors, Barnes & Noble said Burkle may team up with another top investor, Aletheia Research & Management, to try to take control of the No. 1 U.S. bookstore chain without paying shareholders a premium.

Aletheia and Yucaipa hold a combined 33.9 percent of the company, according to Thomson Reuters data, roughly equivalent to the stake held by Riggio, his family and senior executives.

Barnes & Noble shares were down 1.3 percent at $14.47 in morning trading on the New York Stock Exchange.

WAR OF THE WORDS

Burkle, who built his fortune through a series of West Coast grocery store buyouts, has long faulted Barnes & Noble’s management, citing the roughly 40 percent decline in its shares’ value in the past year, and accused Riggio of packing the board with relatives and cronies.

In addition to himself, Burkle nominated Stephen Bollenbach, chairman of KB Home and a former Hilton Hotels Corp CEO and Time Warner board member; and Michael McQuary, CEO of Wheego Electric Cars Inc.

Barnes & Noble on Wednesday questioned Burkle’s competence, saying in Wednesday’s letter that Burkle and his proposed directors had been part of “some of the most spectacular technology and corporate failures in history.”

The company blamed Burkle for being part of the Yahoo board that rejected Microsoft Corp’s 2008 bid. It also faulted Bollenbach’s role in the 2000 merger between America Online and Time Warner, calling it “possibly the worst” deal ever.

“Do not let Burkle destroy Barnes & Noble,” the company urged shareholders.

A Yucaipa spokesman did not respond to a request for comment.

Last week, Barnes & Noble nominated David Golden, a partner in investment firm Revolution LLC, and David Wilson, chief executive of the nonprofit organization that runs the Graduate Management Admission Test, saying they would add “significant technology and financial experience.”

Barnes & Noble on Tuesday reported a deeper-than-expected first-quarter loss as it incurred legal costs in its battle with Burkle and said the fight would hurt its full year results. It has also been contending with years of declines in physical book sales.