The federal government in Berlin and the states where Opel has plants are only prepared to offer financing aid if Canada's Magna wins the battle for the carmaker, a senior ally of German Chancellor Angela Merkel said.

Magna, a Canadian auto parts maker, is competing with RHJ International for control of the German unit of General Motors, which is selling the brand in return for state support it needed after filing for bankruptcy protection in June.

The states with Opel plants and the federal government are in agreement: we will support a decision with guarantees only if GM opts for Magna, Dieter Althaus, premier of the eastern state of Thuringia, where Opel has a plant, told the Handelsblatt newspaper.

Merkel reiterated on Thursday that Germany preferred Magna's bid over that of RHJ, but acknowledged that GM could have a different view on the matter.

RHJ's competing bid would shrink the German automaker, while Magna plans to expand its reach to return to profitability. Magna's co-CEO Siegfried Wolf told Reuters on Thursday that his company and its Russian partner Sberbank had reached an agreement in principle with General Motors management over a contract to buy 55 percent of Opel.

(Reporting by Brian Rohan; Editing by Hans Peters)