Italy's highest court on Thursday threw out the long-running corruption trial of David Mills, a former lawyer of Prime Minister Silvio Berlusconi, in a move that casts doubt over whether a related corruption trial facing the Italian leader will reach a conclusion.

Messrs. Mills and Berlusconi have been facing separate, but related, corruption trials. At the heart of both cases is a $600,000 payment Mr. Berlusconi allegedly made to Mr. Mills in the late 1990s, allegedly so the lawyer would give false testimony to help the Italian billionaire and his Fininvest media empire during two trials of Mr. Berlusconi at the time. Mr. Berlusconi was acquitted in one of those trials; in the other, the statute of limitations ran out, and the case was dropped.