The Supreme Court of Canada has set aside the acquittal of a man charged with sexually assaulting a developmentally-challenged woman and ordered a new trial.

The case centres on whether the woman, who has the mental age of a three- to six-year-old, was competent to testify against the accused.

The trial judge said she wasn't competent and the Ontario Court of Appeal agreed.

But in a 6-3 decision, the high court says the judge set the bar too high.

The majority said witnesses with mental disabilities may testify as long as they can understand and answer the questions and offer an oath or at least a promise to tell the truth.

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

"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. 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."

The minority argued that standard is too low.

The now 26-year-old woman, whose identity is protected by a publication ban, was allegedly repeatedly sexually assaulted by her mother's partner at the time. She was 19 years old when the alleged assaults occurred.

The woman told her special education teacher about a "game" that she and her mother's former partner used to play together, which involved the man touching her. She later repeated this statement to police, indicating the man touched her breasts, genital area and buttocks underneath her pyjamas. She also said it happened many times.