Can a bank make more money by becoming less creditworthy? That is the curious conclusion to be drawn from an accounting quirk that has been a feature of recent results by several large UK banks.

Barclays and Royal Bank of Scotland this week reported hundreds of millions of pounds in gains on the carrying value of their own debt. These profits helped partly to offset the heavy losses the banks had suffered on complex debt securities. For RBS, the £812m gain on its debt helped to more than halve the reported pre-tax loss.