Barclays has revealed it is facing a £50 million fine over claims it acted "recklessly" in its multibillion-pound bailouts from Qatar in 2008.
The Financial Conduct Authority (FCA) accused it of agreeing £322 million of secret payments to Middle Eastern investors to secure their support for cash calls totalling more than £5 billion at the height of the financial crisis.
Barclays, which contests the FCA's findings, said the fees relate to advisory services over five years.
It is also being probed by the Serious Fraud Office and regulators in the US, and admitted it does not know how much the final cost will be.
Barclays was warned about the potential fine on Friday and told shareholders on Monday in a prospectus document for a rights issue that will tap investors for another £5.8 billion to plug a £12.8 billion hole in its finances.
The FCA ruling follows its £290 million penalty last year for rigging the Libor interbank lending rate.