A British unit of GMAC Financial Services LLC was fined $4.6 million by the U.K.'s financial regulator for charging unfair fees on late mortgage payments.

GMAC-RFC Ltd. may have to refund 46,000 customers as much as $13 million in addition to the fine, the Financial Services Authority said Thursday.

From October 2004 through November 2008, the company was too quick to issue foreclosure notices and overcharged customers in arrears, the FSA said.

As public anger with lenders mounts after a bailout that is saddling taxpayers with potential liabilities equivalent to a year's economic output in the United Kingdom, the FSA is taking a harder line on financial companies. The regulator said in June that it was opening enforcement investigations into four firms.

"Whilst our arrears charges were in line with the market, in hindsight, we fully accept that for certain fees our estimates of the costs were not proportionate to the additional administration actually required," GMAC-RFC, of Bracknell, England, said. It said it would refund customers who incurred the charges, plus interest.

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