The Supreme Court of Canada has ordered a new trial in a sexual assault case involving a mentally disabled witness, and has given her the right to testify in court.

The case from Ontario involves a then 19-year-old disabled woman who was allegedly assaulted by a man living with her mother.

The woman, whose identity is protected by a publication ban, was determined to have the mental capabilities of a child three to six years old.

That led the accused to challenge whether she was capable of giving credible testimony in court, and the trial judge excluded this evidence.

Friday's ruling, in a 6-3 decision, set aside the man's acquittal, and ordered a new trial.

It also ruled that people with mental disabilities no longer have to undergo a competency test in order to testify.

Trial judge made 'fundamental error'

Chief Justice Beverley McLachlin wrote in the ruling that to set the bar too high for testimony from people with disabilities would permit violators to sexually abuse victims with impunity.

"Sexual assault is an evil. Too frequently, its victims are the vulnerable in our society — children and the mentally handicapped. Yet rules of evidence and criminal procedure, based on the norm of the average witness, may make it difficult for these victims to testify in courts of law," McLachlin wrote in the ruling, on behalf of the majority.

"The challenge for the law is to permit the truth to be told, while protecting the right of the accused to a fair trial and guarding against wrongful conviction."

'Sexual assault is an evil. Too frequently, its victims are the vulnerable in our society — children and the mentally handicapped.'—Chief Justice Beverley McLachlin

The court was asked to consider whether people with intellectual disabilities must demonstrate an understanding of the concept of a "promise to tell the truth" as a precondition to testifying.

At a preliminary inquiry in 2006, the complainant was found to be a competent witness under the Canada Evidence Act. But the issue of her competence arose again at trial.

The defence questioned her competency and she was subjected to a test.

She had no problem answering questions about her life but had trouble answering questions about truth, lies and promises, the CBC's Alison Crawford reported.

In the end, the trial judge excluded her evidence and the accused was acquitted.

An appeal of this decision was filed to the Supreme Court.

In Friday's ruling, the chief justice wrote that the trial judge made a "fundamental error" in applying the Canada Evidence Act regarding the testimonial competence of adult witnesses with mental disabilities.

Victim told teacher about 'hugging game'

The accused had been living with the victim's mother, along with the victim and her sister from 2000 to 2004.

The victim told her teacher about a "hugging" game that she played with her mother's boyfriend, which resulted in the police being contacted.

Someone also found a photograph of her bare breasts in the trunk of the man's car, CBC News reported.

During her recorded interview with a police officer in 2005, she demonstrated that the game involved the touching of her breasts, genital area and buttocks underneath her pyjamas. She also said that this happened "all the time."