The survey of Mexican forensic physicians conducted by the Boston-based Physicians for Human Rights found that up to 40% of Mexico's 13,000 federal prisoners had complained of being physically abused.

"Torture and ill treatment of detainees continues to be significant problem in Mexico, said Michele Heisler, the author of the study, published in the Journal of the American Medical Association.

The findings were based on questionnaires filled out anonymously by the 184 federal and state doctors, during the survey carried out with the support of Mexico's Attorney General.

Long-term problem

About half of the canvassed forensic physicians reported that torture or ill treatment remained a severe problem for detainees in Mexico.

According to their survey responses, between 1,600 and 5,000 detainees said they had been tortured.

The physicians also said they had documented between 285 and 1,090 cases of ill treatment.

However, some doctors said it was difficult to accurately track a problem that has long plagued Mexico's criminal justice system.

They said that Mexico's police officers had devised ways to minimise the physical evidence of violence, and also admitted that they simply were not trained to act as human rights watchdogs and had no appropriate equipment.

The authors of the survey said that the Mexican authorities would need to retrain forensic physicians in order to stamp out systematic abuses.

"As important, they need autonomy and procedural safeguards within a broader judicial system that enforces respect for human rights," Ms Heisler said.

Mexican President Vicente Fox has promised that Mexico will respect human rights and adhere to international treaties banning torture.