An activist investor on Thursday urged Supervalu Inc. to sell one-third of its grocery stores and portions of its real estate and to bring in new leaders.

In a letter to Supervalu’s board of directors that it released publicly, Blackwells Capital LLC in New York dubbed the company’s financial performance “abysmal” and noted its shares have lost 95 percent of their value over the past decade.

“We believe the board must urgently change direction and guide management to a plan that can improve the company and its performance,” the investment firm said in the letter. “This status quo is untenable, and shareholders cannot stand idly by and accept the ongoing value destruction.”

Supervalu, based in Eden Prairie, is one of the nation’s biggest wholesalers and also operates about 200 grocery stores, chiefly in the Midwest. Its biggest is Cub, the market leader in the Twin Cities.

Supervalu’s real estate, Blackwells Capital said, is worth far more than the company’s $615 million market capitalization, and it wants the company to start selling some of it.

Blackwells Capital, which added shares in recent months to become one of Supervalu’s largest shareholders, also recommended that the company sell about 30 percent of its stores, though it didn’t specify which ones. It said the company should emphasize innovation, including “delivery and meal preparation services,” in the stores it keeps.

The firm also said Supervalu should quickly finish the process of hiring a new chief financial officer — its former CFO, Bruce Besanko, left in June for the same job at Kohl’s — and implement regular dividends and a share buyback plan.

“We take seriously all input from our shareholders,” Supervalu said in a statement issued after the Blackwells Capital letter was released.

“We addressed several of these matters in our recent earnings call held last week and in previous earnings calls,” the company said in its statement. “As we have publicly stated, we continue to evaluate and execute on various initiatives as part of our efforts to transform Supervalu and create value for our shareholders.”

The company’s shares closed up more than 3 percent Thursday. In July, the company declared a 1-for-7 reverse split to avoid being delisted from the New York Stock Exchange.

Chief executive Mark Gross, who joined the company last year, has beefed up Supervalu’s larger business in food wholesaling, including making two acquisitions of distributors this year and selling its Save-A-Lot retail chain last year.

The investment firm said it has been talking with Supervalu management about its ideas and called the dialogue “constructive.” But it added, “We have grown increasingly frustrated with the company’s share price performance and the lack of clear steps Supervalu’s leadership has demonstrated a willingness to take in order to unlock value and position the company for ongoing success.”

The investment firm did not threaten a proxy battle but said it wanted to directly talk with Supervalu’s board of directors.

“In an industry under pressure, Supervalu has tremendous structural advantages and an opportunity to define, and act upon, a corporate strategy that can create long-term shareholder value,” Blackwells Capital said in the letter. “Without active and immediate change, we believe the opportunity will be lost and shareholders will continue to pay the price.”