Royal Bank of Scotland (RBS), the UK's second-biggest banking group, is asking shareholders for an extra ?12bn to shore up its finances.
The rights issue was announced as part of a trading update and is one of the largest seen in UK corporate history.
The firm also announced a write-down of ?5.9bn before tax, following its exposure to the credit markets.
BBC business editor Robert Peston said other banks may well follow suit, to reinforce their financial foundations.
Mr Peston also said RBS would "retain more capital in its balance sheet to meet the risks of default by borrowers than it had been doing".
Richard Hunter, head of UK equities at Hargreaves Lansdown Stockbrokers, said: "RBS may have lost the element of surprise as the news was well flagged, but will nonetheless benefit from being first to the plate as this announcement will inevitably lead to cash calls from others".
Analysts speculate that Barclays and HBOS could also be possible candidates for rights issues.
Writer : BBC