The private-equity firm Permira has teamed up with American Express Co. to bid for Royal Bank of Scotland Group PLC's Global Merchant Services division, a person familiar with the situation said Monday.

The two are joining the buyout firms TPG Inc., Warburg Pincus LLC, CVC Capital Partners and a consortium made up of Bain Capital LLC and Advent International Corp. in the auction.

Canada's largest payment processing firm, Moneris Solutions, is also participating in the bid, which is in the due diligence phase. Final offers for the unit, which provides credit card payment processing services, are due at the end of May.

CVC Capital Partners has also partnered with the French information technology company Atos Origin and the private-equity firm Welsh Carson Anderson & Stowe to bid for the RBS assets, the person said. Welsh Carson focuses on the information and business services, and health-care sectors.

According to the source, RBS has indicated that private-equity firms should partner with companies in the information technology industry to make running its operations easier.

Worldpay, the largest part of the Global Merchant Services business, has operations in more than 40 countries, catering to clients including small and midsize enterprises.

The auction has attracted numerous bidders, especially private-equity firms that could use Worldpay's stable cash stream to pay interest on loans taken out to fund the acquisition.

A deal could include vendor financing, with the bank agreeing to provide a loan to the buyer. In addition, the chosen buyer will also be able to decide whether RBS should keep a 20% stake in the division.

The auction is just one of several disposals being forced on the bank as a condition of its accepting government rescue funding during the financial crisis. The U.K. government retains an 83% holding in the bank.

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