AT&T Inc. and Verizon Wireless, thebiggest U.S. mobile carriers, are planning a venture to displace credit and debit cards with smartphones, posing a new threat to Visa Inc. and MasterCard Inc., three people with direct knowledge of the plan said.

The partnership, which also includes Deutsche Telekom AGunit T-Mobile USA, may work with Discover Financial Services andBarclays Plc to test a system at stores in Atlanta and threeother U.S. cities that would let a consumer pay with thecontactless wave of a smartphone, the people said. The carriershave been searching for a chief executive officer.

The trial would be the carriers’ biggest effort to spurmobile payments in the U.S. and supplant more than 1 billionplastic cards in American wallets. Smartphones have encroachedon tasks ranging from Web browsing to street navigation and nowmay help the phone companies compete with San Francisco-basedVisa and MasterCard, the world’s biggest payments networks.