Opel workers are preparing mass protests if its parent General Motors fails to pick Canada's Magna as the buyer for the European carmaker, a labor leader said ahead of a news conference scheduled for Thursday.

We will then tomorrow with many thousands of people go to Eisenach ... and will symbolically protect the factory from access with a chain of people, Klaus Franz said on German television station ZDF.

GM has sent a negotiator on Opel back to Berlin after a watershed two-day GM board meeting, people familiar with the proceedings said on Wednesday.

It was not immediately clear what action the GM board had decided on for Opel after spending the past month weighing the merits of selling the European unit against the cost of keeping it.

The German government has promised to back the Magna bid with billions of euros in state guarantees. Brussels-listed RHJ International has a rival bid that GM management has said would be easier to implement but which Berlin rejects.

Franz said he had not been informed about any decision on Opel's future taken by the GM board but added that GM had vowed to inform him, the Opel trustees and the German government of any decisions as soon as possible.

A news conference is planned in Berlin for around 8 a.m. EDT, he said.

Franz said that Opel's 25,000 workers in Germany could demand the 4.2 percent wage hike from last year's collective bargaining agreement that they had previously said they would waive for five years.

($1=.6898 euros)

(Writing by Maria Sheahan; Editing by Greg Mahlich)