AT&T faces a lawsuit in federal court after promoting what some rivals and consumers have derided as “fake 5G” technology.

The suit, filed by Sprint late Thursday night, accuses AT&T of marketing that “deceives consumers” into thinking AT&T already offers a next-generation, 5G wireless network when, in fact, customers are still using an upgraded version of 4G LTE.

“The significance of AT&T’s deception cannot be overstated,” Sprint said in its complaint, adding that AT&T’s promotional effort has resulted in lost sales for Sprint and risks confusing the public. AT&T didn’t immediately respond to a request for comment.

AT&T in recent weeks has begun replacing the “LTE” symbol at the top of some smartphones with a new icon that says “5G E,” which stands for “5G Evolution.” The company has said the label reflects the fact that AT&T is on the pathway to rolling out a mainstream 5G network, which could eventually support download speeds up to 100 times faster than the current standard.

But AT&T has yet to switch on such a network for smartphones, prompting Verizon, T-Mobile and Sprint to pile on with tweets, full-page ads in print media and other statements condemning the rhetoric. All four companies are racing against each other to field the nation’s first true 5G wireless network, and in its lawsuit, Sprint said AT&T is trying to gain an “unfair advantage” with its allegedly misleading marketing.

AT&T chief executive Randall Stephenson told CNBC Friday morning that the company feels "very comfortable" with the "5G E" moniker.

“We’ve done our [homework] around how we characterize this,” he said.

Sprint reached out to AT&T before filing the lawsuit, asking the carrier to suspend its marketing. But Sprint was rebuffed, according to Craig Whitney, a lawyer representing Sprint in the case.

“They somewhat predictably came back and said, ‘Thanks but no thanks, we’re going to proceed with our marketing,’ " said Whitney, a partner at Frankfurt Kurnit Klein & Selz. "They’ve doubled down on it.”

Sprint is requesting that the U.S. District Court for the Southern District of New York block AT&T from continuing to use the label “5G E” or any related variant of the label.

At the Consumer Electronics Show in January, John Donovan, the chief executive of AT&T Communications, defended the practice.

“If I now occupy beachfront real estate in our competitors’ heads, that makes me smile,” he said. “Every company is guilty of building a narrative of how you want the world to work. And I love the fact that we broke our industry’s narrative two days ago, and they’re frustrated and gonna do what they’re gonna do.”