* La Caixa may sell 5-8 percent of Gas Natural this year
* Wants to remain a leading shareholder, source says
* Repsol also studying Gas Natural stake sale (Adds Caixa comment, analyst comments, potential new shareholders)
By Jesús Aguado and Tracy Rucinski
MADRID, Sept 11 (Reuters) - Spain's Gas Natural Fenosa is facing a major change in its shareholder base for the first time as La Caixa and Repsol consider future stake sales in the latest unwinding of Spanish corporate cross-holdings.
Financial group La Caixa and oil major Repsol have controlled the company since it was created from the merger of several gas groups in 1992 and the two major shareholders oversaw two decades of strong international expansion.
On Wednesday, a source close to La Caixa said the bank could take advantage of any potential sale of Repsol's 30 percent stake in the gas group, tipped by the oil major in July, to sell a small part of its own 35-percent holding.
Repsol and Caixa's decades-old shareholders' pact says that if the two group have less than 50 percent of Gas Natural or if one cuts its stake to below 15 percent, the agreement is broken.
"La Caixa may sell between 5 percent and 8 percent of Gas Natural before the end of the year, while maintaining its position as a leading and controlling shareholder," the source with direct knowledge of the matter said.
Gas Natural's market capitalisation is about $20 billion.
A La Caixa spokeswoman said it had no plans for a stake sale right now and was committed to being a majority shareholder.
La Caixa shares jumped 6.4 percent. Gas Natural's were flat.
"If La Caixa, a stable shareholder even in the worst of times, is really considering selling at a time when the other large shareholder also wants out, it may have detected enough buying interest so as to make the deal interesting," Kepler Cheuvreux analyst Jose Porta said.
Repsol is considering disposing of its Gas Natural stake because the agreed sale of its liquefied natural gas business to Royal Dutch Shell, due to close this year, diminishes the holding's strategic rationale.
But as a steady dividend-generator, Repsol management has said it is not in a hurry to sell and would prefer for any future deal to be in agreement with La Caixa and Gas Natural.
Repsol has not yet hired an investment bank to sell the stake, though financial sources have said that the oil major is weighing different options for the holding, including a sale to financial or industrial investors.
As for La Caixa, the group would likely use potential capital gains from a Gas Natural stake sale to shore up funds ahead of stricter capital requirements for European banks due to be implemented from 2014.
Spanish banks are under pressure to sell stakeholdings in order to shrink their balance sheets. Bankia has sold stakes in technology company Indra and airline International Airlines Group.
La Caixa, which has the largest portfolio of corporate shareholdings among Spanish lenders, will be under scrutiny in the next round of European banking stress tests early in 2014.
Gas Natural's No. 3 shareholder is Algeria's state-owned energy firm Sonatrach with 4 percent, and the two companies are partners in the Medgaz pipeline that runs from Algeria to Spain.
Sonatrach may be a potential buyer, according to newspaper Expansion, as well as France's GDF Suez, China's Sinopec and Mexican magnate Carlos Slim, whom media reports say has agreed with La Caixa to buy 3 percent of Gas Natural.
Gas Natural is due to present a new business plan this year. (Additional reporting by Carlos Ruano; Editing by Louise Ireland)