VANCOUVER -- Social workers recklessly disregarded their obligation to protect children who were sexually abused by their father, even granting him unsupervised visits during which he molested his toddler, a British Columbia judge has ruled.

B.C. Supreme Court Justice Paul Walker has delivered a scathing ruling in favour of a mother who sued the province after the Children's Ministry failed to investigate her kids' reports of sexual abuse and believed she was mentally ill.

Walker found the province liable for sexual abuse suffered by the toddler while in ministry care and ordered the director of child protection to pay special costs to the mother.

"The director and certain ministry social workers acted well outside their statutory mandate and the duty to protect children," he wrote in a 341-page decision.

"A key feature of the director's conduct in this case is that she and many of her agents approached the case in front of them with a closed mind, having concluded at a very early stage, before the children were interviewed, that there was no merit to the sexual abuse allegations."

Minister of Children and Family Development Stephanie Cadieux did not immediately respond to a request for comment.

The judgment details the mother's "unyielding" efforts over several years to persuade the ministry to act on her children's allegations.

The mother, identified only as J.P., and father have four children born between 2002 and 2008.

In 2009, the father was arrested for assaulting J.P. and their five-year-old daughter and uttering death threats. He then filed for divorce and sole guardianship of the children.

Soon after, J.P. told a social worker she was concerned that the father may have sexually assaulted their youngest child, who was a year old at the time.

The social worker led her to believe that the ministry would conduct an investigation. Instead, he called the officer in charge of the Vancouver Police Department's sexual offences unit to disparage her motives, mental health and veracity of her concerns, the ruling says.

While the Vancouver Police investigation was underway, J.P. notified the ministry that the three older children were reporting that their father had sexually abused them. The children gave detailed accounts of what occurred, according to the judgment.

The father has denied all allegations and has never been charged.

Later in 2009, Walker issued an order that required the father to be supervised during visits with his children. Days later, the ministry seized the children from their mother and after a few months allowed the father to have unsupervised visits.

A trial was held starting in October 2011 to deal with the custody and apprehension issues. For the first two months, the director of child welfare supported the father's claim for custody. However, she abruptly reversed her position in March 2012.

Walker ruled later that year that the father had sexually and physically abused his three eldest children and gave J.P. sole custody.

She filed her lawsuit against the province while the custody trial was underway. Walker's decision released Tuesday is the result of that lawsuit.