An Ivey Business School economist has sounded the alarm about a dramatic spike in the number of temporary foreign workers as London's economy struggled and unemployment was on the rise.

And Liberal Leader Justin Trudeau weighed in on the issue as well.

From 2006 T0 2012 the number of foreign workers in London increased by more than 60 per cent - from 1,100 to 1,800.

In Windsor, the increase was 150 per cent, from 600 to more than 1,500 workers.

"If it's true they're coming in with specialized skills we don't have in southwestern Ontario, then why aren't we training people with those skills?" Economist Mike Moffat asks.

He mentioned the large pool of skilled automobile workers who are unemployed.

A long list of employers given fast track approval of the temporary foreign workers program in 2012 includes companies such as Canadian Tire, Caterpilla, Ellis-Don and the London Health Sciences Centre.

Moffat says there's a risk the program could push down wages. "If companies are allowed to bring in temporary foreign workers instead of hiring (local) workers than it's a bigger pool of workers."

The issue was the subject of a heated debate in the House of Commons on Tuesday, when Trudeau accused the Conservatives of contributing to joblessness in southwestern Ontario by allowing companies to hire foreign help.

"In Windsor, the number of unemployed workers has risen by 40 per cent while the number of foreign workers in the city has grown by 86 per cent," Trudeau said. "Unemployment in London has risen by 27 per cent while the number of foreign workers has increased by 87 per cent."

In two letters obtained by The Canadian Press -- one from Conservative MP Kellie Leitch, now labour minister, and Alberta colleague Blake Richards -- posed even further embarrassments for the government on what's become one of its most vexatious files.

In a letter written in April 2012 to Transport Minister Denis Lebel, Leitch told of an Air Canada pilot in her riding who "expressed concern regarding the hiring of foreign crews and pilots who are driving down the salaries of Canadian pilots as well as contributing to the unemployment of Canadian pilots."

In his response, Lebel referred Leitch to Diane Finley, then the human resources and skills development minister, and Jason Kenney, then minister of immigration.

Richards, meantime, wrote to Finley in late 2009, raising similar concerns about CanJet's hiring practices.

"At a time when many people are having difficulties finding employment, I am sure you can appreciate why some pilots would be upset that their colleagues have been overlooked by CanJet," he wrote.

Data compiled by Employment Minister Jason Kenney's department indeed shows that a slew of temporary foreign workers have been hired in recent years in areas struggling with joblessness, including the Maritimes and southwestern Ontario, and in sectors where there is no lack of domestic candidates.

Kenney's office issued a statement challenging the notion that temporary foreign workers contribute to joblessness rates, citing Statistics Canada findings.

"Statistics Canada clearly stated that, 'The effect of temporary foreign workers on the employment estimates is negligible,' representing two per cent of overall employment," spokeswoman Alexandra Fortier said in an email.