Part-nationalized Royal Bank of Scotland posted a third-quarter loss as it boosted its provision for compensating customers who bought payment protection insurance and took a hefty charge related to its debt.

The company, 82 percent owned by British taxpayers, set aside another 400 million pounds ($644 million) Friday to compensate customers who were mis-sold insurance, raising the total provision to 1.7 billion pounds.

An accounting charge of 1.5 billion pounds on the value of the bank's own debt knocked the bottom line too, while a year ago the result was flattered by an even larger gain on the value of own debt. The charge is an estimate of the gain or loss the bank would realize if it bought back its debt.

As a result, RBS reported a loss of 1.4 billion pounds ($2.2 billion) for the July-September quarter, compared to a profit of 1.2 billion pounds a year earlier.

Accounting rules though obscured the underlying trend. Group operating profit of 1.05 billion pounds was up from 2 million pounds a year ago, and income rose 6 percent to 6.4 billion.

"Underlying performance has already improved enough to be generally comparable to peers," said Chief Executive Stephen Hester.

RBS shares opened 2.1 percent higher at 293.2 pence in London.

"There is no doubting the immensity of the task RBS has faced in executing its turnaround plan, nor indeed the progress made so far," said Richard Hunter, head of equities at Hargreaves Lansdown.

The bank though which was at the heart of Britain's banking crisis of 2008 and eventually bailed out, still has a way to go.

Hester said the bank is in the later stages of a five-year restructuring plan but that there is more to do, "including an emphasis on dealing with reputational issues now that the Bank's safety and soundness has advanced so well."

Investec analyst Ian Gordon said he foresaw trouble ahead for RBS, including its possible involvement in the LIBOR-fixing scandal and the sale of interest rate swaps to borrowers.

During the quarter, RBS completed the flotation of 35 percent of its Direct Line insurance unit. The European Commission has ordered RBS to dispose of its entire interest in Direct Line by the end of 2014 as a condition for taking bailout money.

RBS is still looking for a buyer for 316 branches, another EU condition, after Santander backed out of a deal last month.

The payment insurance scandal has hit other big British banks. Part-nationalized Lloyds Banking Group raised its total provision by 1 billion pounds to 5.3 billion pounds on Thursday, and Barclays earlier boosted its provision by 700 million pounds to 2 billion pounds.