But Juan Bautista Mendoza said that a further sentence reduction is possible if the women co-operate as witnesses against co-conspirators.
Michaella McCollum, 20, from Dungannon, Co Tyrone, and Melissa Reid, 20, from Lenzie, near Glasgow, have a court hearing on Tuesday where they are expected to be interrogated by a judge.
Each faces drug-trafficking charges and could be charged with up to 15 years in prison.
A lawyer for McCollum said after the women's August 6 arrest at Lima's international airport that they were coerced with threats of violence by a gang of up to 15 armed men, some of whom trailed them to Peru from Spain.
"I was simply repeating what she had told the police during the questioning," Peter Madden said yesterday.
He said he could not yet say whether McCollum, who has both Irish and British citizenship, would plead guilty or go to trial.
Mr Mendoza said that if both women plead guilty they could be sentenced to prison in less than a month's time. He said otherwise their trial could last six months.
The prosecutor said a new law enacted two weeks after the women's arrest eliminated sentence reductions for good behaviour for people convicted of drug trafficking.
Previously, parole was possible after 32 months for drug couriers who pleaded guilty.
The women were arrested attempting to board a flight to Madrid with the cocaine hidden in packages of mayonnaise, said Mr Mendoza.
He said he did not believe that they were coerced into carrying the drugs or in any way threatened.
According to Peru's national prisons institute, 90% of the 1,648 foreigners in the country's jails are either sentenced or awaiting trial for drug trafficking.