Prosecutor Juan Rosas said that he will ask for a new hearing to give Melissa Reid, 20, of Lenzie in East Dunbartonshire and Michaella McCollum, 20, from Dungannon, Co Tyrone, the chance to offer a more complete confession.
Mr Rosas said that the women have not accepted full responsibility for the charges against them.
The women pleaded guilty to drug trafficking charges behind closed doors in a Lima court on Tuesday night.
They expected to face six years and eight months in prison without possibility of parole when sentenced next week.
Sources said the women had taken full responsibility for the drug trafficking.
But both women, who had been working in Ibiza, previously claimed they were forced to carry drugs by Colombian drug lords who kidnapped them at gunpoint.
Now prosecutor Mr Rosas said they need to explain why they initially claimed they were coerced by a gang of armed men.
He said: "A new statement is required by the women and they will have to admit not only that they came here to traffic drugs but also that they did not tell the truth with respect to them being kidnapped, they have to make a full admission of responsibility for all of the charges brought by the prosecution."
The prosecution maintains that McCollum and Reid made up a story about being kidnapped and threatened into attempt to smuggle the drugs, weighing 24lb, out of Peru on August 6.
Mr Rosas added: "If they continue to maintain this incredible version of events then the prosecution is not going to accept them taking advantage of a quick conclusion of proceedings."
Reid previously said she was planning to plead guilty in a bid to get home as soon as possible.
She is reported to have said: "I am really scared about what I am about to do but I am also relieved that there could be a light at the end of the tunnel.
"After a lot of thought and advice from my lawyer I am going to go in front of the judge and admit I was in possession of the drugs and that I went to Peru to pick up drugs to take to Spain.
"I did it under duress, and I am glad I do not have to say I accepted money to do it."
Their lawyer Meyer Fishman said he could not comment until they were sentenced.
According to Peru's national prisons institute, 90% of the 1,648 foreigners in the country's prisons are either sentenced or awaiting trial for drug trafficking.