Canadian workers at General Motors began voting Sunday on their union's latest contract concessions, outlined in a deal announced after two weeks of negotiations.

The agreement reached late Thursday would slash annual labour costs by more than $8,000 per worker, but wages and pension benefits are protected.

Under the deal, the company is also committed to addressing a shortfall of more than $6.5 billion in the pension plan, the Canadian Auto Workers union says.

Workers in the Ontario communities of Windsor, Woodstock and St. Catharines voted on the agreement Sunday while workers in Oshawa will vote Monday.

The third tentative contract agreement between GM and the CAW within the past year comes as the company continues to restructure to avoid collapse.

The federal and Ontario governments have told the two sides that major cost concessions are needed before the automaker is eligible for $6 billion in federal and provincial loans.

The company faces a May 31 deadline to present its restructuring plan.

Union leader Ken Lewenza on Friday said a key victory is the company's willingness to restructure its troubled pension plan.

GM Canada had a pension deficit of about $4.9 billion as of November 2007, but reports say it ballooned after financial markets crashed last year.

Specific details of the overall deal were not immediately known, but Lewenza said it delivers a $15 to $16 reduction in the average per-hour wage of GM's Canadian workers on top of a previously negotiated $7 cut.