Erste Bank shares were down 10 percent by 0913 GMT, the worst STOXX 600 faller, while Raiffeisen Bank was down 5.5 percent.

Erste Bank owns Banca Comerciala Romana, the biggest bank in Romania. Raiffeisen Bank has 6 percent of its assets in Romania, it said in an investor presentation in November.

The proposed tax would hit earnings of Austrian lenders Erste and Raiffeisen Bank as well as French lender Societe Generale, KBW analysts said in a note in which they estimated an impact of between two to seven percent on earnings per share.