A jury found three members of an Afghan family guilty of killing three teenage sisters and another woman in what the judge described as "cold-blooded, shameful murders" resulting from a "twisted concept of honour", ending a case that shocked and riveted Canadians.

Prosecutors said the defendants allegedly killed the three teenage sisters because they dishonoured the family by defying its disciplinarian rules on dress, dating, socialising and using the internet.

The jury took 15 hours to find Mohammad Shafia, 58; his wife Tooba Yahya, 42; and their son Hamed, 21, each guilty of four counts of first-degree murder, which carries an automatic life sentence with no chance of parole for 25 years.

After the verdict was read, the three defendants again declared their innocence in the killings of sisters Zainab, 19, Sahar 17, and Geeti, 13, as well as Rona Amir Mohammad, 52, Shafia's childless first wife in a polygamous marriage.

Their bodies were found on June 30 2009, in a car submerged in a canal in Kingston, Ontario, where the family had stopped for the night on their way home to Montreal from Niagara Falls, Ontario.

The prosecution alleged it was a case of premeditated murder, staged to look like an accident after it was carried out. Prosecutors said the defendants drowned their victims elsewhere on the site, placed their bodies in the car and pushed it into the canal.

Ontario Superior Court Judge Robert Maranger said the evidence clearly supported the conviction. "It is difficult to conceive of a more heinous, more despicable, more honourless crime," Mr Maranger said.

"The apparent reason behind these cold-blooded, shameful murders was that the four completely innocent victims offended your completely twisted concept of honour ... that has absolutely no place in any civilised society."

In a statement following the verdict, Canadian Justice Minister Rob Nicholson called "honour" killings a practice that is "barbaric and unacceptable in Canada".

Prosecutor Gerard Laarhuis welcomed the verdict. "This jury found that four strong, vivacious and freedom-loving women were murdered by their own family in the most troubling of circumstances," Mr Laarhuis said outside court. "This verdict sends a very clear message about our Canadian values and the core principles in a free and democratic society that all Canadians enjoy and even visitors to Canada enjoy," he said to cheers of approval from onlookers.