Some of the Catalan leaders on trial would be found guilty of charges of sedition and misuse of public funds but not of the more severe charge of rebellion, the source told Reuters.
The verdict is expected to be made public next week, most likely on Monday, the source said.
A Supreme Court spokeswoman declined to comment.
The public prosecutor had sought the longest prison term, 25 years, for Oriol Junqueras, former deputy leader of the Catalan regional government. But the court plans to sentence him to 13-15 years in jail, the judicial source said.
Major Spanish newspapers including El Pais, La Vanguardia and El Mundo have published similar information about the sentence, also citing sources.