AT&T Inc. and Verizon Wireless must let smaller competitors use their networks for mobile Internet service under rules approved today by a divided Federal Communications Commission.

The agency voted 3-2 with Democrats approving and Republicans dissenting to require large wireless carriers to strike commercial agreements with smaller carriers. Such agreements are voluntary now.

The rule extends to data traffic the network-sharing arrangements that already are mandatory for voice calls. The measure was opposed by Verizon, the largest U.S. wireless provider, and Dallas-based AT&T, which would vault to No. 1 if it completes its proposed $39 billion purchase of Deutsche Telekom AG’s T-Mobile USA Inc.

“Roaming deals are simply not being widely offered” and the requirement will spur investment and promote competition, said Julius Genachowski, the agency’s chairman.

“The commission simply does not have the legal authority” to adopt today’s rules, said Robert McDowell, a Republican commissioner who voted against the measure.

Data-roaming agreements let wireless customers use their devices to connect with Internet sites and e-mail outside their home areas, just as voice roaming lets them connect calls as they travel. Data services are growing in importance as customers increasingly turn to Web-enabled smartphones and tablet devices, such as Apple Inc.’s iPad.

Commercially Reasonable Terms

The FCC order approved today requires carriers to strike agreements on commercially reasonable terms. Carriers unable to reach agreements may appeal to the FCC.

AT&T and Basking Ridge, New Jersey-based Verizon Wireless opposed the measure. Carriers have scores of roaming agreements so there’s no need for regulation, and the agency lacks authority over data services, the companies said in FCC filings.

Small carriers have had trouble reaching agreements with AT&T and Verizon, Steven Berry, president of the Rural Cellular Association, said in an interview. The Washington-based group represents almost 100 companies, according to its website, and members include U.S. Cellular Corp. and Atlantic Tele-Network Inc.

The agency also voted 5-0 to lower rates that telephone companies pay to attach lines to utility poles. The step is part of a set of policies for extending the reach of broadband, or high-speed Internet service.

Verizon was down 22 cents to $37.639 at 12:09 p.m. in New York Stock Exchange trading. AT&T dropped 15 cents to $30.32 at 12:09 p.m.