VATICAN CITY — A Vatican court on Saturday sentenced the pope’s former butler, Paolo Gabriele, to 18 months in prison for leaking confidential documents to a journalist in one of the most serious breaches of vaunted Vatican secrecy in modern history.

The court found Mr. Gabriele guilty of theft and remanded him to house arrest. A Vatican spokesman, the Rev. Federico Lombardi, said Saturday that it was “very likely” that the pope would pardon Mr. Gabriele, who had tended to the pope’s personal needs for six years.

Mr. Gabriele, 46, remained impassive as the chief judge, Giuseppe Dalla Torre, pronounced the sentence “in the name of His Holiness, Pope Benedict XVI, gloriously reigning,” from a wood-paneled room in the Vatican City tribunal, housed in a palazzo behind St. Peter’s Basilica.

The verdict capped one of the most embarrassing episodes in recent Vatican history after a tell-all book based on dozens of the documents leaked by Mr. Gabriele revealed accusations of financial misdeeds within the Vatican, as well as infighting and widespread tensions.