Citigroup, seen as a potential rival bidder for ABN Amro, has effectively ruled itself out of the running after joining the roster of banks advising Barclays on its takeover talks with the Dutch bank.

A source at Citigroup confirmed the Wall Street bank is working for Barclays. The development means Citigroup cannot launch a bid for ABN Amro without stepping down from its role with Barclays.