Labor unions at General Motors Co.'s Spanish plant will call for strike action Tuesday after rejecting a revised offer by Magna International Inc. (MGA) on the future of its Opel manufacturing plant, a Spanish union official said Monday.

Ana Sanchez said unions had rejected Magna's plan - which features smaller production cuts of Opel's Meriva and Corsa models than Magna initially offered - in part because the offer didn't include details on potential job cuts.

Unions have called a plenary meeting Tuesday to vote on the potential strike, she said.

Magna had initially told Spanish unions to expect around 1,700 job cuts at Opel's Zaragoza plant in northeast Spain, out of a total of 7,500 workers, with plans to move part of the production to Germany. That number was later revised down to 1,350 workers.

Magna has reached provisional agreements with unions in Germany, the U.K., Poland, Austria and Belgium.

General Motors Co. is expected to complete the sale of a majority stake in its Opel and Vauxhall brands to Austrian/Canadian car-parts maker Magna and Russian bank OAO Sberbank (SBER.RS) in the coming days.

The sale faces potential regulatory hurdles. The European Union's competition commissioner, Neelie Kroes, said last week there were "significant indications" that Germany had violated E.U. state aid and market rules by making EUR4.5 billion in state aid for Opel contingent on Magna winning the bid.

-By Christopher Bjork, Dow Jones Newswires, 34 913958123, christopher.bjork@dowjones.com