Employers in hard-hit regions of Canada have been hiring temporary foreign workers despite an abundance of domestic job-seekers, government data indicates, while at least two Conservative MPs have privately sounded alarm bells about the besieged federal program.

Temporary foreign workers were the subject of a heated debate Tuesday in the House of Commons, when Liberal Leader Justin Trudeau accused the Conservatives of contributing to joblessness in southwestern Ontario by allowing companies to hire foreign help.

(What is the Temporary Foreign Worker Program? Read The Globe's easy explanation)

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A recent report by the C.D. Howe Institute, a non-partisan public policy think-tank, suggested the program has also spurred joblessness in Alberta and B.C.

"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 a letter to Liberal MP John McCallum, auditor general Michael Ferguson suggested he was open to conducting a review of the program. Both the Liberals and the NDP have been calling for a probe by the auditor general.

"Let me assure you that we are aware of the current debates concerning the program," Ferguson wrote, adding he would be asking the government "for their information in planning for future audits."

But it was two different letters — one from Conservative MP Kellie Leitch, now labour minister, and Alberta colleague Blake Richards — that posed even further embarrassments for the government on what's become one of its most vexatious files.

In an April 2012 dispatch 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."

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Lebel referred Leitch to other ministries.

Richards, meantime, wrote to Diane Finley in late 2009, raising similar concerns about CanJet's hiring practices. Finley was then the minister of human resources and skills development.

"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.

Questioned about the letters during question period, Kenney thanked his Conservative colleagues for the information and suggested the government had cracked down on the use of temporary foreign workers in the aviation sector.

"We appreciate such input," Kenney said. "It has helped to inform our tightening up of this program."

But data compiled by 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.

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In 2012, for example, the government granted positive labour market opinions for 375 temporary foreign workers in Cape Breton, a region of Nova Scotia that had a 17.5 per cent unemployment rate a year ago.

"They always just want to talk about the employers who are abusing, but at the end of the day this government has failed to take responsibility for the fact that they are the ones that approve the LMOs," said Jinny Sims, the NDP's immigration critic.

"They're saying it has nothing to do with us when it has everything to do with them."

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.

In his spring report released Tuesday, the auditor general said Statcan's top survey on job vacancies is too vague and doesn't provide details on the precise locations of labour shortages within the provinces and territories.

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Its industry classifications are also too broad and don't provide enough information about jobs that need to be filled, he added.

An employment insurance report by Employment and Skills Development Canada has been ridiculed for including data from Kijiji, an online classified jobs site, to determine labour needs. The practice has since been dropped.

The temporary foreign worker program has ballooned from about 100,000 people in 2002 to as many as 338,000 now. In 2013 alone, Ottawa approved approximately 240,000 temporary foreign workers.

Earlier Tuesday, Kenney and various Conservative MPs defended the government's handling of the program and employers who hire temporary foreign workers in sectors and regions with legitimate labour shortages.

Alberta MP Chris Warkentin accused Trudeau of demonizing employers who have tried but ultimately failed to find domestic employees.

He pointed to McDonald's restaurants in Grand Prairie, Alta., that can't fill dozens of job vacancies despite offering wages higher than the prevailing market rate.

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Kenney has temporarily banned restaurants from accessing the program amid a spate of abuse allegations. He's expected to announce a new round of rule restrictions soon, including efforts to beef up the auditing powers of federal inspectors.