Shares in the Franco-Belgian banking group, Dexia, have plummeted for a second day following concerns over its exposure to the Greek debt crisis.
They fell by another 37% in early trading on Tuesday after falling 10% on Monday, but rallied back to 21% down.
Dexia called an emergency board meeting on Monday night.
The governments of France and Belgium, which are joint shareholders in Dexia, moved to guarantee its debts, after finance ministers also met on Monday.
The BBC reports the commitment raised questions over the Belgian government's own solvency.
Belgium's 10-year cost of borrowing jumped from 3.7% to 3.8% in bond markets on Tuesday.
Separately, the French and Belgian central banks also stated that they "fully support" Dexia, indicating that they will ensure it does not run out of cash.
The bank is to be reorganised, with problematic loans being moved into a "bad bank" that will be more closely supervised by the two governments.
The BBC reports Dexia recently comfortably passed stress tests by regulators of all the major European banks.
Dexia's exposure to Greek government debt totals 3.4 billion euros ($US4.5 billion). Its total exposure to Greece is 4.8 billion euros. It has already written off 21% of its Greek debts.
Dexia received a 6 billion euro bailout at the height of the financial crisis in 2008.
Shares in other banks were also down on Tuesday.
In France, Natixis ended 9.4% lower. BNP Paribas fell 5.2% and Credit Agricole 6.5%.
In Britain, Barclays dropped 7.6%.