The parent company of The San Diego Union-Tribune said Wednesday that it has agreed to sell its flagship newspaper to a private equity firm for an undisclosed price.

Copley Press Inc. agreed to sell the dominant newspaper in the nation's eighth-largest city to Platinum Equity, based in Beverly Hills, Calif., the newspaper reported on its Web site. Copley put the newspaper up for sale in July 2008.

Louis Samson, a principal at Platinum Equity, called the newspaper "a good fit."

"The Union-Tribune is more than a business. It's an institution in San Diego," Samson said. "But it faces enormous challenge in a period of tremendous upheaval for the newspaper industry. We will bring a strong operational focus that helps ensure the Union-Tribune not only survives in this market, but thrives."

Like much of the newspaper industry, the Union-Tribune's fortunes have sagged as readers and advertisers have migrated to the Internet.

Publisher Gene Bell told employees in January that the Union-Tribune's advertising revenue had fallen 40 percent since 2006. He said then that the newspaper faced a "challenging" environment in its search for buyers, forcing unpaid furloughs and other cost-cutting measures.

The Union-Tribune won a Pulitzer Prize in 2006 for its reporting on the bribery scandal that landed former Republican Rep. Randy "Duke" Cunningham in federal prison.