Shares in Bankia, taken over by the Spanish state because of a mass of bad loans, plunged Thursday after a report that clients had withdrawn over one billion euros in the past week.
The bank, Spain's fourth-largest, and the government swiftly moved to calm concerns, but shares in the bank continued a deep plunge that began last week when the state intervened.
Bankia shares closed 14.08 percent lower at 1.422 euros. Madrid's IBEX-35 index of leading stocks was down by 1.11 percent at the close.
Shares in the bank had plunged earlier Thursday by 27.49 percent to 1.2 euros, less than one-third of their value of 3.75 euros when the shares were listed on July 20, 2011.
Newspaper El Mundo reported that Bankia managers told the board that in the past week, the bank lost a "similar amount" of deposits to the 1.16 billion euros ($1.5 billion) withdrawn by clients in the first quarter of the year.
The newspaper said the new numbers were presented during a Bankia board meeting on Wednesday.
Spain announced on May 9 that it would take a controlling 45-percent stake in Bankia by nationalising its parent group Banco Financiero de Ahorros (BFA), the latest in a series of moves to stabilise the troubled banking sector.
Bankia moved to calm deposit holders' concerns on Thursday. In a statement, it reiterated that it was solvent and said in a drop in deposit levels was "seasonal".
"In these turbulent economic times, Bankia's activity in recent days has been basically normal," added Bankia's chairman Jose Ignacio Goirigolzarri in a recorded interview.
"Our customers can feel very confident and safe because Bankia is a tremendously solid entity. It's not just us who work for Bankia that say that, but also the Bank of Spain and the government."
The Bankia statement added: "Indicators of how the balance sheet is evolving show that the balance of deposits will not register substantial changes in the coming days."
The government also issued a message of calm.
"It is not true that a run on deposits is currently taking place in Bankia," junior economy minister Fernando Jimenez Latorre told a news conference Thursday.
"Bankia had a problem with its capital requirements, a problem that has been resolved with the contribution and commitment of the state to cover all its financing needs."
Bankia, created in 2010 from a merger of seven savings banks, had 112 billion euros in deposits from clients at the end of the first quarter.
Bankia had the sector's largest exposure to the property market at 37.5 billion euros at the end of 2011, of which 31.8 billion euros were classed as problematic, according to figures from the Bank of Spain.
Spain's property sector collapsed in 2008 after a decade-long boom.
Last week, Bankia said it would set aside 4.7 billion euros -- by far the highest amount among Spanish banks -- to cover potential losses in case loans to the property sector went bad, in line with a government reform plan.