UPDATE 2-Banks consider 4 bln euros for Nabucco gas pipeline

* Three international banks start due diligence for loans

* Loans to Nabucco could total 4 billion euros

* Financing and construction schedule slips slightly

(Adds detail, quotes)

By Pete Harrison

BRUSSELS, Sept 6 (Reuters) - Plans for the giant Nabucco gas pipeline to bring Central Asian gas to Europe took a step forward on Monday, when the consortium announced the involvement of three international financial institutions.

But the banks will not make a final decision on whether or not to lend up to 4 billion euros ($5.2 billion) until sometime next year, and the schedule of the long-delayed project appears to have slipped further.

The $10 billion project aims to help wean Europe off its dependency on Russian gas by transporting up to 31 billion cubic metres of gas a year from the Caspian region to an Austrian gas hub via Turkey and eastern Europe.

The Nabucco consortium said it had signed an agreement with the European Investment Bank (EIB), the European Bank for Reconstruction and Development and the World Bank's International Finance Corporation, whereby the three banks will start due diligence for loans that could reach 4 billion euros.

"We are at the beginning of the appraisal process -- it is not predetermined," EIB executive Thomas Barrett told reporters. "But the fact we are ready to identify a potential 4 billion euros can be taken as a very serious remark."

The European Commission, which also plans to grant up to 200 million euros to the project, welcomed the move and said the pipeline will help bolster Europe's energy security.

CEE Bankwatch, a campaign group that monitors spending by international financial institutions, said public money would be better spent on renewable energy rather than more fossil fuels.

"The EIB is supposed to be the EU's financial powerhouse and it should follow EU climate commitments," the group said in a statement.

CRITICAL YEAR

This year is seen as critical for Nabucco, which faces tough competition from Russia's planned South Stream pipeline via the Black Sea, and from two much cheaper European projects to import gas via Turkey -- the Trans-Adriatic Pipeline and ITGI.

Other challenges are continuing weak European gas demand due to the economic crisis, and increasing competition with China for Central Asian supplies.

The project appeared to have been pushed back slightly again, with the start of construction now seen in 2012, compared to predictions as recently as March that it would kick off by the end of next year.

Nabucco Managing Director Reinhard Mitschek said the timing of contracts for space in the pipeline would depend upon how soon gas buyers and suppliers have completed their own negotiations.

The banks are expected to make their decision sometime in 2011.

"I'm confident we'll close the whole financing package in Q3 or Q4 (of 2011)," he told Reuters. "In 2012, we'd start construction and in 2015 the first gas would flow."

Nabucco's shareholders include Hungary's MOL MOLB.BU, Romania's Transgaz TGNM.BX, Bulgaria's Bulgargaz, Turkey's Botas, Germany's RWE (RWEG.DE) and Austria's OMV (OMVV.VI). (Editing by James Jukwey)