Royal Bank of Scotland, the UK bank that is majority-owned by taxpayers, relied on a mortgage-trading unit in the US for nearly half of the profits made by its global banking and markets division last year, according to figures released by the bank.

The division almost single-handedly kept RBS in the black, with operating profits of £3.5bn, or $5.4bn using the average exchange rate in 2010. Overall, group operating profits were £1.9bn in 2010, after the bank took a £5.5bn loss on its "non-core business".