American Express Co has teamed up with UK private equity firm Permira to bid for Royal Bank of Scotland's payment-processing arm, the Financial Times reported in its Monday edition.

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, which includes the WorldPay business, the FT said.

Citing people familiar with the situation, the paper said the move follows suggestions that RBS would prefer a sale to a strategic partnership capable of running the payment- processing business.

RBS, 84 percent state-owned, is being forced by European competition authorities to sell WorldPay and other assets, including a network of 318 branches and its commodities business, to compensate for billions of pounds of taxpayer money injected during the financial crisis.

Industry sources have said the unit is expected to fetch 2.5 billion to 3 billion pounds ($4 billion to $4.8 billion).

American Express, Permira, CVC and RBS could not immediately be reached for comment.

Last month, sources told Reuters that five bids, including four from private equity firms, had been selected to move through to the second round.

(Reporting by Caroline Copley; Editing by Jan Paschal)

United kingdom Software Royal Bank of Scotland Citigroup