Mexico's Supreme Court has said a Frenchwoman jailed for 60 years for kidnap should not be freed immediately.
But the court allowed for the possibility of a retrial of Florence Cassez, 37, saying there were violations in the case.
Ms Cassez was arrested in 2005 at a ranch near Mexico City where several victims were being held, but denies knowledge of the abductions.
The French government says it regrets the court's decision.
The review was requested by a Supreme Court judge, Arturo Zaldivar, who says Ms Cassez was denied her consular rights and the right to be presumed innocent after her arrest.
In a motion presented to the court, he said police violated her rights by failing to notify the French consulate and failing to present her to investigative officials.
Judge Zaldivar said the irregularities included the fact that Mexican police made Ms Cassez take part in a televised staging of the raid.
In its decision, the Supreme Court review panel decided not to release Ms Cassez immediately.
But it accepted that there had been police misconduct and violations of Ms Cassez's rights after her detention, leaving open the possibility that the case could need to be reheard.
Florence Cassez maintains that her only connection with the case was that she was the girlfriend of one of the kidnappers.
The affair has raised tensions between France and Mexico; a high-profile cultural event in Paris was cancelled last year over the issue.
In France, Ms Cassez is widely considered the victim of a miscarriage of justice.
The French President, Nicolas Sarkozy, has questioned her conviction and called for her release.
Speaking in Paris, Ms Cassez's mother Charlotte said she was "sad, disappointed and shocked" at the decision by the Mexican court.
"We must once more keep up the fight," she told reporters.
But the BBC Mexico correspondent, Will Grant, says many Mexicans are indifferent to her case or angry that she receives what they see as special treatment as a foreigner.
And several members of the Mexican government, including the President, Felipe Calderon, has urged the court panel to uphold justice for victims.
Mexico has one of the world's highest kidnap rates, with victims sometimes murdered even after ransoms have been paid.