OTTAWA  The governments of Canada and Ontario told the Canadian Auto Workers on Thursday to offer General Motors of Canada additional concessions in contract talks.

The union signed an agreement in March that reduced G.M.’s Canadian labor costs by an estimated 7 Canadian dollars an hour ($5.96). But in April, the auto workers agreed to 19 Canadian dollars an hour ($16.17) in savings for Chrysler Canada which, unlike its parent company, is not under bankruptcy protection.

Dalton McGuinty, the premier of Ontario, suggested to reporters that the union would have to offer a deal to G.M. Canada along the lines of its pact with Chrysler Canada before the government’s new deadline of May 15.

Ken Lewenza, the president of the union, said he had been told that without additional concessions, the two governments would not extend aid to G.M.’s Canadian unit. That, he added, would force the liquidation of the General Motors subsidiary.