La Caixa filed documents Thursday saying it had "informal contacts" with buyers for the stake. Spanish construction group Sacyr Vallehermoso said last week it was looking to drop its 20 percent share in the privately owned Spanish refinery to pare losses from debt-backed financing.
Regulatory papers filed by Repsol Friday indicated it "was aware of the existence of contacts between some of its biggest shareholders and third parties," the Financial Times reported.
Sacyr said in September it was looking for buyers for its stake in Repsol. Russian gas monopoly Gazprom reportedly expressed interest in the acquisition, though Russian media reports last week denied those claims.
Several members of the Spanish government voiced opposition to the deal, though Prime Minister Jose Luis Rodriguez Zapatero appeared to warm to the acquisition, saying it was between private companies.
"The government must respect the companies and negotiations for the incorporation of other shareholders," he said.