Belgian union leaders rejected a plan from General Motor's European arm Opel to rescue a plant in Antwerp, Belgium, arguing the proposal would result in too many job cuts.

Opel said on Tuesday it had put forward a new plan under which an investor would be sought to take over the Antwerp plant, with Opel possibly maintaining a minority stake.

The plant, it said, would be significantly downsized, but would continue to build the Astra TwinTop while also trying to attract business from third parties.

Union leaders said this entailed a steady winding down of production to some 4,000 to 5,000 cars per year with a workforce of just 400 envisaged at the end of 2011, down from 2,600.

"It's just too little," said Eddy De Decker of the ACV union who attended talks with Opel in Germany on Tuesday.

He said the unions had instead demanded that Opel withdraw an existing plan to close the plant by the end of June and give workers more time for an alternative to be found.

"We need a transition period until the end of 2011 or into 2012 in which we are able to produce a meaningful number of cars," he said, adding that should be around the 80,000 to 85,000 units the plant was currently making per year.

De Decker said the unions were awaiting Opel's response.

No one from Opel was immediately available for comment