Germany pressed suitors of General Motors unit Opel to improve their offers and a fourth bidder from China emerged on the eve of a top-level meeting to decide the fate of the carmaker.

Economy Minister Karl-Theodor zu Guttenberg told reporters on Tuesday after meeting Fiat Chief Executive Sergio Marchionne in Berlin that the Italian carmaker had presented a serious offer for Opel but that rival bidders Magna and RHJ International remained in contention.

In an unexpected twist, China's Beijing Automotive Industry Corp (BAIC) also submitted an offer, a spokesman for the Economy Ministry said, potentially turning a fiercely contested three-way competition into a four-way battle for control of a carmaker that traces its roots in Germany back to the 19th century.

We received such a statement of interest this evening, said a spokesman for the ministry, when asked whether BAIC had made an offer for Opel.

GM will have the final word on who buys Opel, but Berlin will play a crucial role in the decision because it is being asked to cough up billions of euros' worth of loan guarantees as part of any deal.

Opel has 25,000 workers at four plants in Germany and the carmaker's fate has become a political hot-potato in Berlin ahead of a federal election on September 27.

Members of Chancellor Angela Merkel's Christian Democratic Union (CDU) are reluctant to see the state spend taxpayer money to save Opel and some are pressing her to allow an orderly insolvency of the firm rather than back a deal with one of the bidders.

Merkel will host a 9 p.m. meeting on Wednesday with the aim of settling on one or more preferred bidders. Senior German cabinet members, leaders from the states where Opel has plants, and top executives from GM and the bidders will attend.

Pressure to close a deal is building ahead of a June 1 restructuring deadline for GM set by the U.S. government, which could lead to a Chapter 11 bankruptcy filing by the Detroit-based carmaker.

GM has said it could have to file for bankruptcy if it cannot retire 90 percent -- or $24 billion -- of its bond debt but an offer to exchange shares in a reorganized company has met with a tepid response.

AGGRESSIVE PUSH

Fiat's Marchionne made an aggressive last-ditch push to address German concerns about his ambitious plan to fold Opel into a transatlantic car empire that would also include U.S. carmaker Chrysler.

He met with Merkel, Guttenberg and Foreign Minister Frank-Walter Steinmeier on Tuesday, but did not come away sounding optimistic about Fiat's prospects.

I'm not sure it is a question of confidence, he told reporters. We are updating people on where we are. I am going to be there tomorrow and we will see what happens.

As Fiat lobbied the government in Berlin, Magna and RHJ were presenting their plans to labor representatives at Opel's headquarters in Ruesselsheim, near Frankfurt.

After those meetings, Opel labor leader Klaus Franz said he believed Magna was the leading contender to get Opel.

But Germany's Frankfurter Allgemeine Zeitung, citing an internal government assessment of the offers, said Berlin had grown skeptical about Magna because it was not investing any of its own capital in Opel and would leave the carmaker with billions of euros in debt.

Guttenberg said insolvency remained an option if the bidders did not make credible commitments to preserve German jobs and showed a willingness to assume greater financial risks.

There's no favorite, he said. Everyone knows that improvements are still necessary.

Outside Germany, GM Europe has plants that employ a combined 15,000 staff in Spain, Poland, Belgium and Britain.

Sweden's Saab, also part of GM's assets, is being sold separately. It said on Tuesday it was in the process of selecting one final candidate from a field of three potential partners lining up to buy it.

(Writing by Noah Barkin; Editing by David Cowell and Matthew Lewis)

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