TORONTO -- Canada's highest court will hear the appeal of four Alberta fathers who were ordered to pay retroactive child support in a case that could dramatically impact the financial obligations of countless divorced or separated Canadian parents.

Should the Supreme Court of Canada uphold the original January ruling, it could result in hundreds of thousands of parents being hit with lump-sum support payments totalling millions of dollars, said Deidre Smith, one of the lawyers leading the appeal.

Despite existing court orders and separation agreements, the fathers were ordered by an Alberta judge to make immediate lump-sum child support payments, some stretching as far back as 1997, to reflect changes in their incomes, Smith said.

One of the appellants "is about as opposite from a deadbeat dad as you can get," Smith said from her Toronto office.

"(He is) being whacked with this significant retroactive award when a trial judge has said, `Hey, buddy, you did everything you are supposed to do."'

The fathers, who are from the Grand Prairie, Calgary, Edmonton and Red Deer areas, can't be named because of a publication ban.

Neither provincial legislation, the federal Divorce Act nor federal child support guidelines require parents to make annual adjustments to their payments, Smith said.

The sudden court demand for tens of thousands of dollars has caught the appellants off-guard, she added.

"They really resent being called deadbeats or being told by the community that they haven't taken care of their kids when they've been paying what the court order or agreement told them what to pay," Smith said.

When new federal payment guidelines were introduced eight years ago, Ottawa included a process by which the government could calculate child support payments based on tax returns and notify parents of required adjustments in pay.

Those guidelines, however, have not been implemented because the provinces have not yet signed on, Smith said.

Virtually anyone who is paying child support under a court order or separation agreement made within the last 15 years may face large amounts of back claims, Smith said, noting that the "vast majority" of Canadian dads earn more after they are separated.

"If they went back and looked at their old orders, there probably would be additional support owing. I've got to think it's in the millions."

Currently, separated or divorced parents receive a court order telling them how much they owe, or work out an agreement with their lawyers, Smith said.

But as child access arrangements change, parents remarry, have more children and incomes fluctuate, the original support arrangement often no longer accurately reflects each parent's situation, she noted.

Mothers and fathers alike often under-report changes in income, day care costs and associated expenses of raising children -- some of the major factors in determining child support, said Stacy Robb, president and founder of Dads Canada.

Some parents make their payments under the table to avoid the costs of returning to court annually for support readjustments, Robb said.

"As a result, some of these people are paying too much money, and others are paying not enough."

One of the divorced fathers involved in the appeal has never earned more than $23,000 a year, and yet he faces a retroactive child-support payment of $10,000, while another faces a payment of $100,000, which is more than half his annual income, Smith said.

The court is expected to begin hearing the appeal between February and May next year.