Melissa Reid, 19, from Lenzie, East Dunbartonshire, and Michaella McCollum, 20, from Northern Ireland, made the behind-closed-doors plea when they appeared before a judge in capital Lima last night.
Sources said the women had taken full responsibility for the drug trafficking.
The two had the option of pleading guilty in exchange for a lower sentence. They were told that if they did they could face a minimum of six years and eight months in prison, with no eligibility for parole.
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."
The pair were caught trying to board a flight from Lima to Spain with 24lbs of cocaine in their luggage last month.
The women's lawyer Meyer Fishman said he could not comment until they were sentenced.
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.