Canada's top court ruled Monday that divorced and separated parents have a duty to report increases in their income when it comes to paying child support.

The decision could affect hundreds of thousands of parents across the country.

The7-0 decision bythe Supreme Courtof Canada means thatformer spouses could face big retroactive support payments. Thecourtleft the door open for lower courts to decide on those payments on a case-by-case basis.

The top court ruled that once a court decides to make a retroactive award, it should generally make the award retroactive to the date when "effective notice" was given to the parent paying child support.Effective notice occurs when theparent receiving support gives any indication child support should be paid, or if itis alreadybeing paid, that the current amount of child support needs to be renegotiated.

However, the court said, when aparent paying support engaged in "blameworthy conduct," such asavoiding orreducing support,the date when thepaying parent's circumstances changed materially will be the presumptive start date of the award.

Monday's ruling stemmed from four child support cases that went to the Alberta Court of Appeal. In those cases, the fathers in each case were ordered to make retroactive payments, ranging from $10,000 to $100,000.

In Monday's decision, the Supreme Court upheld the decisions against two of the fathers and ruled in favour of the other two fathers.

"I think the court has been very clear that all support payers have an obligation to pay child support in accordance with their income at the time," Dee Smith,lawyer for the four Alberta fathers who challenged support awards imposed on them, told CBC News.

"The income goes up, the child supportshould be going up as well."

Smith said the topcourt has also beenclear that everyone now has an obligation to let the support recipient know when there has been a change in the income, at least annually.

"I think the court has narrowed substantially the circumstances in which significant retroactive awards are going to be made," she said.