AT&T Inc. and Verizon Wireless, the biggest 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 AG unit T-Mobile USA, may work with Discover Financial Services and Barclays Plc to test a system at stores in Atlanta and three other U.S. cities that would let a consumer pay with the contactless wave of a smartphone, the people said. The carriers have been searching for a chief executive officer.

The trial would be the carriers’ biggest effort to spur mobile payments in the U.S. and supplant more than 1 billion plastic cards in American wallets. Smartphones have encroached on tasks ranging from Web browsing to street navigation and now may help the phone companies compete with San Francisco-based Visa and MasterCard, the world’s biggest payments networks.