A law professor says Toronto has the highest number of undocumented workers in Canada.
Audrey Macklin of the University of Toronto says temporary immigration, especially for employment purposes, can often lead to undocumented workers.
Macklin also says the most-populous city in the country is a good place to hide.
She told Toronto's Community Development and Recreation Committee on Thursday that the 'Four In/Four Out' rule imposed by the federal government in April 2011 will lead to a dramatic rise in the number of undocumented workers by April 2015.
The rule puts a four-year limit on temporary work permits, after which the worker must leave the country or, as many do, live under the radar illegally.
Macklin says it's too early to know the number of workers who will be affected by this rule.
However, she says other countries with similar guest worker policies have seen a significant increase in undocumented workers.
"It's a law that manufactures illegality," she said. "They are turned into illegals."