Royal Bank of Scotland Group PLC is in talks to sell its credit card payment processing unit, valued at as much as $3.8 billion, to Advent International Corp. and Bain Capital LLC, two people with knowledge of the matter said Friday.

The Edinburgh company may announce the sale of RBS WorldPay before reporting earnings on Aug. 6, said the people, who declined to be identified because the details were private. An RBS spokesman declined to comment.

Officials at Advent International and Bain, both of Boston, declined to comment.

RBS is selling assets including WorldPay, 318 U.K. bank branches and its insurance division after receiving $70 billion of funding from the U.K. government during the credit crisis, more than any other bank in the world.

Other private-equity companies also made offers, including TPG Capital and Clayton, Dubilier & Rice LLC, people familiar with the matter said.

WorldPay operates in more than 40 countries, providing credit card payment processing services to clients including small and midsize businesses. RBS said WorldPay is one of the 10 largest processors of online payments in the U.S.

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