Yahoo has filed suit against Facebook in a California court, accusing the social networking giant of infringing on 10 patents held by the internet pioneer.
"Yahoo has invested substantial resources in research and development through the years, which has resulted in numerous patented inventions of technology that other companies have licensed," the company said in a statement.
"These technologies are the foundation of our business that engages over 700 million monthly unique visitors and represent the spirit of innovation upon which Yahoo! is built," the Sunnyvale, California-based company said.
"Unfortunately, the matter with Facebook remains unresolved and we are compelled to seek redress in federal court," Yahoo said. "We are confident that we will prevail."
Facebook, which was founded in 2004, a decade after Yahoo, expressed disappointment with the Yahoo move.
"We're disappointed that Yahoo, a longtime business partner of Facebook and a company that has substantially benefited from its association with Facebook, has decided to resort to litigation," a Facebook spokeswoman said.
"Once again, we learned of Yahoo's decision simultaneously with the media," the spokeswoman said. "We will defend ourselves vigorously against these puzzling actions."
Yahoo, in the suit filed in US District Court for the Northern District of California, accused Facebook of infringing on patents in several areas.
"For much of the technology upon which Facebook is based Yahoo got there first and was therefore granted patents by the United States Patent Office to protect those innovations," Yahoo said in the suit.
"Yahoo's patents relate to cutting edge innovations in online products, including in messaging, news feed generation, social commenting, advertising display, preventing click fraud and privacy controls," the suit said.
"Facebook's entire social network model, which allows users to create profiles and connect with, among other things, persons and businesses, is based on Yahoo's patented social networking technology," it said.
Once seen as the Internet's leading light, Yahoo has struggled in recent years to build a strongly profitable, growing business out of its huge web presence and global audience.
Scott Thompson, formerly head of mobile payments firm PayPal, became chief executive at Yahoo at the beginning of January promising urgent action to turn the company around.
Thompson came in after many months of turmoil over the company's direction, including deadlocked talks over possibly selling off valuable assets in China and Japan.
Two weeks after Thompson was recruited, Yahoo co-founder and former chief executive Jerry Yang resigned from the board of directors.
A few weeks later the chairman and three other directors said they would step down, opening the way for Thompson's agenda.