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Royal Bank of Scotland, one of four big British banks to be downgraded by Moody's Investors Service, is complaining that the rating is "backward-looking."

Part-nationalized RBS was downgraded Thursday along with HSBC, Barclays and Lloyds Banking Group.

RBS said Friday that the downgrade was "backward-looking and does not give adequate credit for the substantial improvements the group has made to its balance sheet, funding and risk profile."

However, RBS says it believes the impact is manageable. Part-nationalized Lloyds also says the drop in its long-term rating would have little effect.

HSBC and Barclays did not immediately react to the rating changes announced late Thursday.

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