Michaella McCollum and Melissa Reid have pleaded guilty to attempting to smuggle 11.5kg of cocaine out of Peru, court officials said.
Ms McCollum, 20, from Dungannon, Co Tyrone, and Melissa Reid, 20, from Glasgow, gave behind-closed-doors pleas when they appeared before a judge in the port town of Callao, near the capital Lima.
The pair took full responsibility for drug trafficking after initially claiming they were forced to board a flight from Lima to Spain with cocaine in food packets hidden inside their luggage last month.
A spokesman for the court in Callao said: "They will automatically have a sixth off from the minimum jail sentence of eight years and will be sentenced to six years and eight months in prison.
"Sentencing will take place on 1 October at a new hearing."
Ms Reid's family has previously said they are working with the British Foreign Office in the hope that the Peruvian authorities will allow her to serve part of her sentence in the UK.
Speaking outside the court in Lima, lawyer Meyer Fishman said he could not comment until the women were sentenced.
Both women, who had been working on the Spanish island of Ibiza this summer, had previously claimed they were coerced into carrying the drugs by Colombian drug lords who kidnapped them at gunpoint.
So far they have been held at the Virgen de Fatima prison in Lima, but court officials said they may now be transferred to the equally tough Santa Monica women's jail.