Juan Bautista Mendoza said the pair, who were arrested at Lima airport, can expect to receive sentences of at least six years and eight months each.
Melissa Reid, from Lenzie, East Dunbartonshire, and Michaella McCollum, from Dungannon, Co Tyrone, would have no eligibility for parole even if they pleaded guilty.
Their families had hoped they would be allowed to serve part of their sentences in the UK.
However, Mr Mendoza said a sentence reduction is possible if the women testify against co-conspirators, but otherwise they could face between eight and 15 years.
McCollum's lawyer Peter Madden said he could not yet say whether she would plead guilty or go to trial.
Ms Reid's father Billy has previously said she hoped to get less than seven years.
They claim to have been coerced by a gang of up to 15 armed men.