Barclays has reported a pre-tax quarterly loss of £47m - compared with a £2.4bn profit over the same period last year.
The statutory loss in the three months to September was driven by increasing costs relating to the mis-selling of payment protection insurance (PPI) and a one-off £1bn charge against the value of the bank's own credit.
The bank had previously said it would set aside a further £700m to cover its costs after customer complaints over PPI continue, taking its total estimated bill to £2bn.
If this charge is not included, Barclays reported underlying pre-tax profits for the third quarter of £1.7bn, compared with £1.3bn last year.
Its investment bank more than doubled its quarterly underlying pre-tax profits to £937m, but its UK retail banking fell 19% year-on-year to £400m.
Barclays, which was rocked by the Libor rate-rigging scandal, also unveiled two new US regulatory inquiries into its conduct.
The bank said the US Department of Justice and US Securities and Exchange Commission were investigating whether its relationships with third parties who help it win or retain business are compliant with the US Foreign Corrupt Practices Act.
It is also being investigated for past power trading in the west of the US.
Barclays' new chief executive, Anthony Jenkins, said the company would end 2012 in a "strong position".
"These results demonstrate that we continue to have good momentum in our businesses despite the difficulties we faced through this period," he said in a statement.
"While we have much to do to restore trust among stakeholders, our universal banking franchise remains strong and well positioned."