A Vatican appeals court on Tuesday sentenced a priest to two and half years in prison for corrupting a minor, the latest chapter of a teen sex abuse case at a school for altar boys in the Vatican City.

As part of the long-running affair, Gabriele Martinelli, a former pupil at the St Pius X pre-seminary and now a priest, was accused of assaulting a fellow altar boy -- years before he joined the clergy.

He and a former rector of the pre-seminary, who was accused of shielding his protege from punishment, were acquitted in October 2021 of charges linked with sex abuse. 

That trial was the first to be held before a Vatican criminal court over allegations of sex abuse or cover-ups within Vatican territory.

At the time, the judges ruled that Martinelli and his accuser did engage in various sexual acts, but there was "lack of evidence" over their non-consensual nature.

The sexual relationship between the pair started in 2007, when Martinelli was 14 and his younger peer 13, and continued until 2012.

The acquittal was appealed, with the charges reformulated as corrupting a minor.

On Tuesday, the appeal judges convicted Martinelli and sentenced him to two and a half years in prison and a 1,000-euro fine, according to the decision seen by AFP.

The appeals court said Martinelli "is not liable for the contested acts up until August 2, 2008, since he was less than 16 years old."

But he was held liable for the acts from August 9, 2008, until March 19, 2009, since he was then considered to have attained sexual maturity, while his victim had not.

"We are certainly very happy," Laura Sgro, the lawyer for the victim, told AFP.

"This is an important day for my client because after so many years of suffering, his hardship and the violence he underwent was actually acknowledged, so it was a very important moment."

"The hope is that this ruling can somehow still give an even deeper reflection on sexual abuse in the Church," Sgro said.

The Saint-Pius X pre-seminary, which hosts boys interested in the priesthood and who help serve mass in St Peter's Basilica, used to occupy a building next to Pope Francis's residence.

The Holy See said in 2021 the institute would be relocated away from Vatican grounds, to be closer a private school in central Rome where the boys take their classes.