Trading in shares in the Spanish lender Bankia has been suspended in Madrid.
The market regulator CNMV said it was “due to circumstances that may affect the normal share trading”.
Bankia is reported to be due to ask the government for a bailout of more than 15bn euros ($19bn; £12bn) after a board meeting later on Friday.
Bankia, which is Spain’s fourth-largest bank, was part-nationalised two weeks ago because of its problems with bad property debt.
Any extra government money would be on top of the 4.5bn euros in state loans that the government converted into shares in the group in the part-nationalisation process.
Shares in Bankia’s parent company Banco Financiero y de Ahorros (BFA) have also been suspended.
Bankia had to reassure its savers last week that their money was safe after a Spanish newspaper reported a run on the bank.
Bankia was created in 2010 from the merger of seven struggling regional savings banks. It holds 32bn euros in distressed property assets.
Spain’s economy minister Luis de Guindos said on Wednesday that the government would pump at least 9bn euros into Bankia but that more would be available if it was needed.
There have been four attempts by Spanish governments to shore up the banking system since the global banking crisis of 2008.
As part of the latest plan, lenders are having to make 30bn euros of extra provisions to cover potential losses on property loans, which comes on top of 54bn euros they were ordered to set aside in February.
The health of Spain’s banking system is key to whether the country eventually needs to seek a bailout itself from the eurozone and the International Monetary Fund.
Spain’s credit rating was downgraded by Standard & Poor’s last month on the basis that it would probably have to take on more debt to support its banks.
Its shares fell 7.4% on Thursday to close at 1.57 euros, which is 58% down from their listing price in July 2011.