WINNIPEG -- A former archbishop who sexually assaulted an altar boy in the 1980s has lost his appeal and is starting his eight-month sentence behind bars.

The Manitoba Court of Appeal said Thursday it agrees with the conviction of Seraphim Storheim and the sentence a lower court imposed. The high court said it wasn't convinced trial Judge Christopher Mainella -- who now sits on the Court of Appeal -- made a mistake when he decided Storheim's evidence wasn't believable.

"It cannot be said that the trial judge applied a different level of scrutiny to the evidence of the accused," the judges wrote. "A review of the decision reveals that the trial judge ... did not hold the accused's evidence to a higher standard than that of the Crown witnesses."

The judges also ruled the sentence fit the crime.

Storheim was found guilty last year of sexually assaulting a boy who had come to visit him in Winnipeg in 1985. Storheim's lawyer, Jeff Gindin, had argued Manitoba's highest court should consider fresh evidence and overturn the verdict because his client didn't get a fair trial.

He argued new photos and documents suggesting the boy was in Winnipeg in 1986, and not 1985, came to light after the verdict and cast doubt on the credibility of the victim and the Crown's version of events.

The judges said they weren't persuaded "it would be in the interests of justice to admit the fresh evidence."

"We're disappointed," Gindin said Thursday. "(Storheim) has had some time to prepare for the possibility. He was prepared for the worst, even though he was hoping for the best.

"The court has the final say so we have to accept it."

Storheim, who had been out on bail, was immediately taken into custody and will begin his sentence separated from the general prison population, Gindin said. The former archbishop could be out in as soon as three months, he added.

Storheim was convicted of sexually assaulting one boy, but he was acquitted on the same charge involving the victim's brother, who also visited on a separate occasion.

The victim testified that during his visit Storheim would routinely walk around naked and would sometimes lie naked on the floor and touch himself. The man testified that another time Storheim touched him and inspected his groin as he sat naked on a bed.

Storheim testified he talked to the boy about puberty and inspected his pyjama bottoms at the request of the boy, but nothing inappropriate took place.

Storheim was an archbishop in the Orthodox Church in America -- the church's highest-ranking cleric in Canada. He was placed on leave when he was arrested in 2010 and he retired following his conviction.

His followers immediately decried the Appeal Court decision on a website dedicated to him.

"All who love and support Archbishop Seraphim are saddened and continue to pray for him," the website said. "It is, however, not a defeat as Archbishop Seraphim remains innocent and faithful to God."

His supporters also urged people to pray for "those who accused him ... They do not know what they are doing."