A Chinese automaker has offered to buy a stake in General Motors unit Opel, challenging a deal from Canadian auto parts supplier Magna, sources said on Friday.

State-owned Beijing Automotive (BAIC) has submitted an indicative, non-binding offer for Germany's Opel, sources familiar with the situation told Reuters, and would make a binding offer by mid-July.

The news comes ahead of a Friday deadline for GM lawyers to submit papers to U.S. federal bankruptcy Judge Robert Gerber who will rule by July 10 on a plan to create a new GM with brands Chevrolet, Cadillac, Buick and truck unit GMC.

Magna, backed by Russian partners, is the frontrunner to buy Opel. Sources told Reuters Thursday that Magna's board of directors wants to approve a business plan for Opel on July 7.

Belgian holding company RHJ International is the third bidder, although it posted a sharply wider full-year loss on Wednesday and analysts are doubtful it has the cash to compete with its rival suitors.

Whoever wins the race for Opel faces a tough antitrust probe, a German government minister said Friday.

When asked whether he counted on Brussels examining any deal for Opel under a lengthier, so-called 'phase II' investigation, German Deputy Economics Minister Jochen Homann told reporters: Yes, clearly.

He declined to specify what issues might concern EU competition authorities since the German government is still awaiting a business plan for the takeover, which he hoped to get by the end of July.

Any business plans submitted will be evaluated by PriceWaterhouse Coopers on behalf of Berlin, since the government will have to offer more state aid after already extending Opel a six-month 1.5 billion euro ($2.1 billion) bridge loan that matures on November 30.

Every day that there is no restructuring costs money, every month that goes by is a lost month, said Homann, the head of the government's Opel task force.

Some support may come from Britain, where GM produces and markets Opel cars under the Vauxhall brand.

The British government said Friday it was prepared to support any deal, possibly in the form of loans, if the terms were right.

I'm not going to discuss the detail of any financial underwriting, but it may involve loans or loan guarantees, Business Secretary Peter Mandelson said. We would obviously have to have interest paid and some security (for the loans).

MAGNA DOUBTS

Speaking at a Handelsblatt auto industry conference, Homann expressed skepticism over Magna's concept for Opel and said that vested interests praising the auto parts supplier's offer as the best left the German government open to extortion.

We will see whether Magna's concept is successful. It is based on theories over which one can heartily discuss...There are justifiable question marks, he explained.

He expressed doubts as to the wisdom of basing a business plan on the boom-and-bust Russian car market and questioned whether Magna could lure other carmakers to utilize Opel's unused production capacity.

Homann, who reports to Minister Karl-Theodor zu Guttenberg, stated no preference for any of the three current bidders.

Homann warned that whoever wants to acquire Opel would have to prepare for a tough EU cartel probe.

The negotiating parties would be well advised to develop a concept that is economically sustainable, fulfils our laws governing loan guarantees and is compliant with EU subsidy rules, he told the conference.

Homann pointed to the chronic structural overcapacities that the EU is concerned about and cautioned that the European Commission has stated a bailout could not distort competition.

Sources familiar with the Opel negotiations have voiced anxiety that Paris might try to install French Economy Minister Christine Lagarde as the next EU Competition Commissioner under a likely second term for President Jose Manuel Barroso.

They fear she might be more critical in part since Opel does not have any major manufacturing operations in France, home to key competitors PSA Peugeot Citroen and Renault, which counts the French state as a large shareholder.

($1=.7133 Euro)

(Editing by Michael Shields and Simon Jessop)

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