Barclays faces a £50 million ($80 million) fine over its 2008 capital raising, as its income sank by £500 million in July and August compared with the same period last year.
The investment bank had previously announced negotiations with the U.K.'s Financial Conduct Authority (FCA) over its £11.8 billion capital raising at the height of the credit crisis. The fundraising with investors in Qatar helped it avoid some of the problems which beset other U.K. banks like Royal Bank of Scotland and Lloyds during the crisis.
An investigation into fees paid over the fundraising - a total of £322 million - which were not disclosed at the time has resulted in the FCA issuing the bank with a warning notice, which Barclays is contesting.
The notice is an indication of what the regulator believes the fine should be. A source at the FCA said that the conclusion of the process was "not imminent."
(Read more: Barclays' rights issue)
Barclays has recently paid hefty fines over the manipulation of Libor by its traders and the mis-selling of both PPI insurance and credit card insurance.
The bank's adjusted income in July and August was £500 million lower than in the same period in 2012, with investment banking in particular performing worse. Barclays described its performance as "significantly below" the same time last year.