Capt Byron Lima Oliva (right) protested the revised sentence

Col Byron Lima Estrada and his son, Capt Byron Lima Oliva, had been serving 30 years each for their involvement.

But the court re-classified them as "accomplices" - a less serious crime.

The bishop was bludgeoned to death soon after presenting a report blaming the army for almost all atrocities during Guatemala's civil war in 1960-1996.

A third former officer, Sgt Obdulio Villanueva, who was also convicted for his involvement in the murder, was killed in a prison riot three years ago.

Ruling protested

The court's ruling on Tuesday came after defence lawyers successfully argued that more than one person cannot be convicted of a killing under Guatemalan law.

The bishop had been a tireless human rights crusader

Capt Byron protested the revised sentence, saying that his prosecution was motivated by politics.

Also on Tuesday, the court upheld a 20-yer jail term against Mario Orantes, the who was the bishop's assistant.

The same court of appeals threw out the convictions and ordered new trials against the four men in 2003. But Guatemala's Supreme Court upheld the original rulings.

Bishop Gerardi was the head of the Roman Catholic Church's human rights office that presented the damning report.

He was bludgeoned to death with a concrete block in the garage of his Guatemala City seminary in April 1998.

At least 150,000 people are believed to have been killed in Guatemala's civil war, and more than 50,000 disappeared.