* One of biggest bankruptcies in Spanish history
* Natixis, Banco Popular and Bankia highest exposure
* Spanish banks already written down billions in losses
* Gecina shares down 4 pct in Paris
(Recasts with confirmation)
By Carlos Ruano
MADRID, Oct 3 (Reuters) - Two Spanish investment firms that own 31 percent of French property company Gecina have filed one of the biggest bankruptcy actions in Spanish history after a bank refused to refinance a 1.6 billion euro ($2.1 billion) loan.
Alteco and MAG Import said in a statement on Wednesday that they were up to date with their payments on the syndicated loan, and the refinancing effort had been supported by other banks involved.
The uncertainty over the fate of the firms' stake in Gecina, which has a market capitalisation of 5 billion euros, knocked the French company's shares down 4 percent to 77.5 euros.
Spanish banks have already written off hundreds of millions of euros in losses on soured real estate investments after a property market crash in 2008 and are now waiting for rescue funds from Europe.
The banks with the highest exposure to the syndicated loan are Popular , Bankia , NCG, France's Natixis and Royal Bank of Scotland , sources said.
One source said Natixis had balked at refinancing the loan. The bank had no immediate comment.
Many businesses related to the property and construction sectors, once the main drivers of the Spanish economy, crumbled after the market crashed. Others have relied on bank refinancing to stay afloat.
One source put Bankia's exposure to the syndicate at 234 million euros, while newspaper El Pais said Popular's was 264 million euros.
Shares in Bankia, which has already been taken over by the Spanish state, fell 3 percent, and Popular's were down 1.5 percent in Madrid trade.
Popular was the largest non-nationalised bank to fail a stress test on Spanish banks last week, forcing it into a 2.5 billion euro rights issue to bolster capital and avoid international aid.
"We understand this (Gecina) will be one of the credits included in the stress test to the sector," Banco Sabadell said in a note to clients on Wednesday.
Gecina, which manages a roughly 11 billion euro portfolio of offices, residential and student housing as well as health facilities, last year launched a 1-billion euro asset sale programme to shore up its balance sheet and reduce its debt.
The company, France's biggest office landlord, underwent a boardroom shake-up in October 2011, when its CEO was dismissed over strategic differences, replaced by Bernard Michel, a former executive at French bank Credit Agricole.
Alteco and MAG Import are owned by one-time real estate magnates the Soler family and Joaquin Rivero, who was also the chairman and shareholder of Spanish real estate developer Metrovacesa .
Metrovacesa is Gecina's other main shareholder, with 26.8 percent, but it is now controlled by Spanish banks after a debt-for-equity swap.
The relationship between Gecina and its Spanish shareholders has been murky from the start. As chairman of Gecina in 2009, Rivero tried to navigate its purchase of a 49 percent stake in another Rivero property firm, Bami, for 108 million euros.
The deal fell through when Gecina's other shareholders rejected it. Rivero is facing corruption charges in France as a result of his attempt to take over one of his own companies through Gecina.
($1 = 0.7731 euros)
(Reporting By Carlos Ruano and Tracy Rucinski; Additional reporting by Sonya Dowsett; Editing by Will Waterman)
Keywords: SPAIN GECINA/BANKRUPTCY