Sprint sued AT&T on Thursday over its use of 5G E branding on phones that are only tapping into advanced 4G LTE.

AT&T started using the 5G Evolution marketing last year, reasoning that it serves as a foundation for true 5G next-generation cellular technology. However, 5G E doesn't mean your 4G phone can actually connect to a 5G network -- causing rivals Verizon and T-Mobile to slam it as "fake 5G." The Sprint lawsuit claims that AT&T is using "numerous deceptive tactics to mislead consumers."

Android phones started displaying 5G E connectivity in January, and it reportedly started appearing on Apple iPhones in recent days.  

Sprint is seeking an injunction to prevent AT&T from using 5G E labels on its devices or advertising, as previously reported by Engadget.

AT&T said it will fight the suit.

"Customers want and deserve to know when they are getting better speeds," AT&T spokesperson Jim Greer said in an emailed statement. "Sprint will have to reconcile its arguments to the FCC that it cannot deploy a widespread 5G network without T-Mobile while simultaneously claiming in this suit to be launching 'legitimate 5G technology imminently.'"

AT&T CEO Randall Stephenson told CNBC in an interview Friday that 5G E is "a critical step" that's offering customers greater speeds as it works towards "ultimate 5G."

Sprint's lawsuit highlights a survey it commissioned that found 54 percent of consumers think AT&T's 5G E is the "same as or better than a 5G network" and 43 percent incorrectly think they can currently buy an AT&T phone that can connect to a 5G network.

Sprint also took issue with AT&T Communications CEO John Donovan's comments at a CES 2019 keynote address last month, where he expressed pride that his company "broke our industry's narrative" with the branding. His comments highlight how aggressive and confusing the carriers' race to 5G has become.

In an emailed statement, Sprint said AT&T is "deliberately deceiving consumers" with the branding.

"AT&T is just like Sprint and all the other major wireless carriers currently operating a nationwide 4G LTE network," Sprint spokesperson Lisa Belot said in the email. "AT&T's deceptive ads have harmed consumers by persuading them to purchase or continue purchasing AT&T's services based on the lie that they are offering 5G."

Sprint filed the lawsuit in US District Court in the Southern District of New York. You can read Sprint's suit here:

First published Feb. 8 at 6:49 a.m. PT.
Update, 8:22 a.m. PT: Adds details from AT&T CEO Randall Stephenson's CNBC interview.

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