American Express Co has teamed up with UK private equity firm Permira to bid for Royal Bank of Scotland's $4 billion payment processing arm, people familiar with the matter said on Monday.

French software firm Atos Origin has also joined the private equity consortium of CVC Capital Partners and Welsh Carson Anderson & Stowe to bid for RBS's Global Merchant Services (GMS) unit, a person familiar with the matter said.

About six firms or consortia are in the hunt to buy GMS, which includes the WorldPay business and is expected to fetch between 2.5 billion and 3 billion pounds ($3.7 billion-$4.4 billion).

RBS, American Express, Permira and Atos Origin declined to comment or could not immediately be reached.

RBS's sale of 318 branches is nearing a conclusion, with Spain's Santander and National Australia Bank now conducting detailed due diligence, sources said.

Spain's BBVA is also on the list of potential bidders but has been asked for more details and it has held talks with NAB over a possible tie-up, according to a person familiar with the matter. It was not clear whether a deal had been reached or how a carve up would work. NAB and BBVA declined to comment.

RBS, 83 percent state-owned, is being forced by European competition authorities to sell WorldPay and other assets as a cost of taking billions of pounds of taxpayer cash during the financial crisis.

RBS and its commodities joint venture with Sempra Energy are in the process of selling their remaining North American assets after selling non-U.S. businesses to JPMorgan for $1.7 billion in February.

The remaining business has drawn a variety of suitors, including independent oil trader Vitol Group, the Wall Street Journal reported on Friday. Other suitors include oil and utility companies and banks with little exposure to U.S. regulation, it said.

Bidders have been teaming up to buy WorldPay, which is likely to be a complex business to separate and run.

Permira and American Express have been working together for a number of weeks, one of the sources said.

Five bids, including four from private equity firms, were last month included to move through to the second round of bidding for WorldPay, sources told Reuters.

In addition to the CVC consortium, they included Canada's Moneris Solutions, TPG , Warburg Pincus and joint suitors Advent International and Bain Capital.

(Reporting by Steve Slater, Simon Meads, Clara Ferreira-Marques and Caroline Copley in London, Cyril Altmeyer in Paris and Judith MacInnes in Madrid; Editing by Jan Paschal and Jon Loades-Carter)