The Guatemalan study, which was never published, came to light in 2010 after Wellesley College professor Susan Reverby stumbled upon archived documents outlining the experiment led by controversial US doctor John Cutler.

Cutler and his fellow researchers enrolled 1,500 people in Guatemala, including mental patients, for the study, which aimed to find out if penicillin could be used to prevent sexually transmitted diseases.

Initially, the researchers infected female Guatemalan commercial sex workers with gonorrhoea or syphilis, and then allowed them to have unprotected sex with soldiers or prison inmates.

Cutler also was involved in a highly controversial study known as the Tuskegee Experiment in which hundreds of African-American men with late-stage syphilis were observed but given no treatment between 1932 and 1972.

The Guatemalan president has called the 1946-48 experiments conducted by the US National Institutes of Health “crimes against humanity” and ordered his own investigation.