Ulster Bank had a third-quarter operating loss of £85 million compared to a profit of £81 million in the year-earlier period, it was announced today.
The worsening losses at Ulster Bank and parent RBS's US retail and commercial business came as impairments rose and operating conditions remained tough.
Impairment losses at Royal Bank of Scotland Group's Irish unit surged to £144 million from £17 million, the Edinburgh-based bank said.
CEO Cormac McCarthy said the bank was still a "core part" of RBS after the group said earlier this week that it would sell off some businesses to offset EU competition concerns.
Earlier this week, Bank of Ireland reported a pre-tax loss of €979 million for the six months ending September 30th compared to a profit of €647 million last year.
RBS more than halved its operating losses to £1.5 billion in the third quarter as impairments fell, but less favourable trading battered investment banking profits.
Part-nationalised RBS said impairment losses totalled £3.3 billion, down 30 per cent from £4.7 billion in the second quarter and were showing signs of levelling off.
But the bank cautioned bad loans were expected to "remain at elevated levels for the next few quarters".
RBS said operating profit for its global banking and markets business shrunk to £375 million in the latest quarter, from £1.1 billion in the second quarter, as "exceptionally strong conditions" seen in the first and second quarters eased.
"I have repeatedly said this is a marathon, not a sprint, and so it is proving," Chief Executive Stephen Hester said of the banks' performance over the past three months.
RBS had warned in August that investors should not expect a "miracle cure" for the battered bank, where investment banking profits, which have lifted the bottom line for most of its rivals, have failed to offset the impact of bad debts.
Earlier this week RBS secured a further £25.5 billion capital injection from the government as part of a deal that will make the terms of a government-sponsored insurance scheme for bad debts more flexible, but will also see dramatic disposals and increase the state's stake to over 84 per cent.
Both RBS and bailed-out rival Lloyds Banking Group agreed to cut back their business to meet EU state aid rules, but RBS was particularly badly hit, forced to sell chunks of its retail bank, its RBS Insurance arm and to shrink its investment bank.
Additional reporting