OTTAWA -- The number of foreign workers in Canada tripled between 2002 and 2012 -- although they still made up less than two per cent of the overall labour force, the parliamentary budget office reported Thursday.

It said more and more foreign workers are filling skilled positions today as the percentage of low-skilled jobs has declined.

The PBO study looked at the role foreign workers played in the economy between 2002 and 2012.

Foreign workers can enter the labour market through either the international mobility program or the temporary foreign worker program, which came under fire last year after allegations surfaced about some employers -- particularly restaurants -- abusing the program.

Employers who want to hire temporary foreign workers must get government approval under what is called a local market impact assessment. The assessment means the government generally knows the skill levels of the workers.

However, almost 70 per cent of foreign workers are exempt from these assessments under the mobility program.

"Since 2002, their number has grown at a faster pace than workers requiring an (assessment)," the report said. "Moreover, in 45 per cent of the cases in 2012, the government was not aware of the occupational skill level of foreign workers, up from 22 per cent in 2002."

The PBO report said 85 per cent of the foreign workers in low-skilled positions were in British Columbia, Alberta and Ontario and were concentrated in smaller communities.

"Alberta's share has increased significantly since 2002, while Ontario's has declined by almost 15 percentage points," the report said. "Interestingly, the relative importance of foreign workers in the labour force appears to be higher in small centres than in large census metropolitan areas."

The study said a significant number of foreigners work on farms, in restaurants or as babysitters or nannies -- jobs on the low end of the pay scale.

Employers don't seem to want to raise wages in these areas, so they are forced to rely on either unemployed domestic workers with few skills or foreign workers, the report said.

But the report said the number of low-skilled Canadians in the work force shrank by 26 per cent between 2002 and 2013.

The characteristics of the foreign work force have also changed since the 2009 recession. Before then, most of the growth was in low-skilled jobs.

"Since then, the number of workers in low-skilled positions has declined by 20 per cent, while the number in skilled positions has increased 20 per cent. As a result, by 2012, the majority of foreign workers were in skilled positions."

The Conservative government introduced new rules in June to limit the number of foreign workers that large- and medium-sized companies are permitted to hire.

It says the changes are aimed at ensuring Canadians are first in line for jobs.

Liberal critic John McCallum said the report confirms that the program has driven down middle class wages and displaced Canadian workers.

"We must restore the program back to its original purpose: to fill acute job shortages when qualified Canadian workers absolutely cannot be found," McCallum said in a statement.