FRANKFURT - Opel's labor force demanded greater say in the process of selecting a future investor for the German carmaker and called on former parent General Motors GM.UL to forgo any right to buy back a stake at a later point.

Labor leaders threatened to boycott efforts to cut costs at the auto maker if they didn't gain a greater say in determining the eventual buyer.

(Such) an investment by the employees is linked to clear prerequisites and demands, labor leaders said in a statement signed by all four works council chairmen in Germany, two regional IG Metall trade union leaders and three other labor representatives.

The labor leaders said no current bidder is prepared to bring in as much financially as the unions and threatened to otherwise not cooperate in reducing structural costs by up to 1.5 billion euros ($2.14 billion) by 2014.

The labor representatives and IG Metall would therefore not make any contributions to the company without a participation in the decision process -- regardless of whichever investor GM might choose.

Labor has heavily favored a consortium of Magna and Sberbank as opposed to financial investor RHJ International.

Opel workers said they should be given a right to veto any potential investor and demanded GM refrain from hopes that it could build a buyback or right of first offer clause into a sale agreement -- one reason why it GM sources have said the company favors RHJ.

It attacked GM for not honoring an contractual agreement to build an SUV based on the Gamma platform, accusing it of planning to finance the transfer of this production to South Korea using expected state loan guarantees.

Instead it referred to its own business plan drafted together with consultants at Management Engineers that foresee 5.7 billion euros in possible savings in Europe's structural costs.

The existing Opel/Vauxhall plants should receive guarantees that rule out production shifts at least until the end of the business plan in 2014, it said.

(Reporting by Christiaan Hetzner)