Private banks will be asked to make an "extraordinary contribution" to the state Deposit Guarantee Fund of €3 for every €1,000 they hold in deposits, said Finance Minister Luis de Guindos on Friday.
That will provide the fund with €1.5 to 2.0 billion ($1.9-2.6 billion) which will then be used to compensate small investors who bought complex and risky "preference shares" sold by now nationalized lenders such as NovaCaixaGalicia and CatalunyaCaixa, he added.
After Spain's decade-long property bubble burst in 2008 sending the economy into a tailspin, savings banks stepped up their marketing of preference shares as they tried to boost their solvency ratios to meet stricter regulatory demands.
But as Spain's economic downturn deepened the value of the preferred shares plunged and it made it effectively impossible to resell them, causing thousands of small investors to lose their savings.
Many small investors say their banks had assured them that the higher-yielding preferred shares were as safe a savings deposit and they have held noisy protests outside of their bank branches to demand their money back.
"This is a real problem because €8 billion worth of these types of products were sold and it affects some 300,000 people," de Guindos said, adding 75 percent of the preferred shares were sold in 2008 and 2009.
"There are preferred shares all over the world but they are sold to specialized investors. In Spain they were sold especially to private investors."
Under the government plan holders of preferred shares will receive regular bank shares in exchange. The Guarantee Deposit Fund will then buy out the shareholders of the now nationalized banks since they do not have traded shares.
Spanish banks' balance sheets were strewn with red ink after a 2008 property crash, forcing Spain to prop them up with €42 billion in rescue loans offered last year by its eurozone partners.
As one of the conditions of extending the rescue funds, Brussels demanded that holders of preference stock share in their banks' losses.