India's Supreme Court on Wednesday banned police and intelligence officials from forcing suspects to take "truth drugs" and lie detector tests during investigations to extract information.
Law enforcement agencies in India have recently relied on the tests, including in the 2006 "House of Horrors" case in which 21 people were murdered in the New Delhi suburb of Noida.
"No individual can be forced and subjected to such techniques involuntarily," the judges said, according to the Press Trust of India news agency. "It amounts to unwarranted intrusion of personal liberty."
The judgment, which followed petitions from several accused, was welcomed by many lawyers including Mumbai-based attorney Majeed Memon who said it "protected the constitutional right of personal liberty".
Manoj Goel, a lawyer representing one of the petitioners, described the verdict as "a historic judgment and a big boost to human rights."
But some police and intelligence officials expressed concern that they would be denied the chance to use essential tools.
"These techniques are used the world over," Maxwell Pereira, a former top officer with New Delhi police, said. "The ruling is a major setback for law enforcement agencies."
In "truth drug" tests, chemicals such as sodium thiopental are injected into suspects or witnesses, lulling them into a semi-conscious state during which they are questioned.
Senior human rights lawyer and activist Colin Gonzalves described "truth drugs" and lie detectors as "a voodoo science, a kind of torture that is grossly misused by the police."
"I am glad the Supreme Court has done this. In India we have many instances of people being coerced into saying things, admitting guilt," he said.
"These methods generate a form of compelled testimony and our constitution prohibits this."
Information gained through the techniques is already inadmissible in Indian courts but Wednesday's ruling states that police cannot use the tests, or investigate any leads provided by them, unless suspects volunteered.