U.S. officials say Huawei Technologies Co. can covertly access mobile-phone networks around the world through “back doors” designed for use by law enforcement, as Washington tries to persuade allies to exclude the Chinese company from their networks.

Intelligence shows Huawei has had this secret capability for more than a decade, U.S. officials said. Huawei rejected the allegations.

The U.S. kept the intelligence highly classified until late last year, when U.S. officials provided details to allies including the U.K. and Germany, according to officials from the three countries. That was a tactical turnabout by the U.S., which in the past had argued that it didn’t need to produce hard evidence of the threat it says Huawei poses to nations’ security.

When telecom-equipment makers sell hardware such as switching gear, base stations and antennas to cellphone carriers—which assemble the networks that enable mobile communication and computing—they are required by law to build in ways for authorities to tap into the networks for lawful purposes.

These companies also are required to make sure they themselves can’t gain access without the consent of the network operator. Only law-enforcement officials or authorized officials at carriers are allowed into these “lawful interception interfaces.” Such access is governed by laws and protocols in each country.