UPDATE 1-RHJ to offer 300 mln eur for Opel majority-sources

* Belgian finance investor to meet Berlin officials Wed

* RHJ, Magna aim to submit offers by Monday -sources

* Magna board members on standby to approve offer -source

(Adds details, background)

FRANKFURT, July 14 (Reuters) - RHJ International (RHJI.BR), a rival bidder to frontrunner Magna (MGa.TO) for German carmaker Opel, plans to offer around 300 million euros ($419 million) for a stake of just over 50 percent, two sources familiar with the matter said on Tuesday.

The proposal from the Belgian holding company "tends toward 300 (million) but is not quite that much," one person told Reuters.

The other source said the equity that RHJ would risk losing was "around 300 million" euros.

A takeover of the General Motors' GMGMQ.PK unit would require the state to assume the rest of the risk by guaranteeing loans worth another 3.8 billion euros, the source said.

RHJ's bid comes in below the 660 million euros that Beijing Automotive (BAIC) plans to invest for a 51 percent stake in Opel along with the Chinese carmaker's request for 2.64 billion euros in guarantees.

Canadian auto parts maker Magna and its Russian backer Sberbank (SBER.RTS) originally offered to invest at least 500 million euros for a combined 55 percent stake. It is expected to ask the state to provide 4.5 billion euros in guarantees.

RHJ Chief Executive Leonhard Fischer is expected to present his plan to high-ranking officials including Jochen Homann, head of the government's Opel Task Force, in Berlin on Wednesday.

Magna has been considered the clear frontrunner since Opel's labour unions have found their bid the best of those offered.

Union support is key if a bidder wants workers' help in cutting 5.7 billion euros in structural costs. [ID:nWEA0509]

Local politicians that govern the states that are home to Opel's four German plants have also supported Magna.

"It is just a diversion," Rhineland-Palatinate premier Kurt Beck said on Tuesday, adding that RHJ did not have a coherent concept for Opel.

Both the Brussels-based finance investor and Magna aim to have a contract, in which all major details are clarified, ready before the beginning of next week, according to several sources.

A person familiar with Magna's plans said that the members of its supervisory board were "on standby" and could quickly convene in order to approve the Canadian auto parts supplier's business plan for a takeover of Opel.

(Reporting by Angelika Gruber, Christiaan Hetzner and Philipp Halstrick; Editing by David Cowell)