It comes after prosecutors in Peru's capital Lima rejected the pleas entered by Melissa Reid, of Lenzie, East Dunbartonshire, and Michaella McCollum, of Dungannon, Co Tyrone.
The pair, both 20, had admitted to attempting to smuggle 24lb of the drug in packets on a flight to Spain from Lima on Tuesday.
They had been told to expect jail sentences of six years and eight months in prison without parole.
However, in a surprise move this has now been thrown out.
Prosecutor Juan Rosas said that he will ask for a new hearing to give McCollum and Reid of Glasgow the chance to offer a more complete confession.
If this is not forthcoming, there is the possibility they will face a trial and, if found guilty, are likely to receive far lengthier terms in prison in Peru.
Mr Rosas said they need to explain why they initially claimed they were coerced by a gang of armed men.
He disputed a statement issued by the court where the women entered their plea that said they had provided information on their co-conspirators.
"The prosecution thinks the charges have not yet been completely embraced. They have simply accepted transporting drugs, but what has not yet been examined is their original version - that they were kidnapped or were transporting the drugs against their will," Mr Rosas said.
"As far as the prosecution is concerned, these citizens were never kidnapped, were never threatened or coerced.
"If they stick to that unbelievable story the prosecution is not going to allow them the benefit of a guilty plea."