Sprint is suing AT&T in federal court over its decision to rebrand some of its 4G LTE networks as “5G E.” The lawsuit, first spotted by Engadget, claims that consumers are likely to confuse AT&T’s so-called “5G Evolution” network with actual 5G, and will incorrectly think that AT&T’s current phones support the new standard. As well as seeking to prevent AT&T from branding anything as 5G E that doesn’t adhere to the agreed 3GPP 5G specification, Sprint is also seeking damages due to the loss of sales it claims it’s suffering as a result of AT&T’s actions.

On Friday morning, Sprint’s outside counsel Craig Whitney told The Verge’s editor-in-chief Nilay Patel that the company is seeking an injunction to immediately stop AT&T from using its 5G E branding while the case plays out.

In its legal complaint, Sprint said that it had surveyed customers and had found that 54 percent of them believed that AT&T’s “5G E” (short for “5G Evolution”) is as fast as, or faster than, actual 5G. Sprint’s CTO has previously said that AT&T’s branding “blatantly misleads” consumers who see “5G E” logos appear on both Android and iOS devices when connected to parts of AT&T’s enhanced 4G network.

“I love the fact that we broke our industry’s narrative two days ago.”

AT&T is using the logo to denote parts of its 4G LTE network that support the faster LTE Advanced and Advanced Pro technologies. AT&T claims that the technology offers speeds of up to twice that of standard 4G LTE, but the 40 Mbps speed of the tech is very similar to what 4G LTE already offers, and falls short of the speeds 5G will one day be able to achieve.

Mobile carriers have been united in their condemnation of AT&T’s 5G E branding, but until now, this has been limited to strongly worded statements and a single well-placed sticker. T-Mobile’s CTO said that AT&T is “duping customers” with the move, while Verizon assured its customers that it “won’t take an old phone and just change the software to turn the 4 in the status bar into a 5.”

AT&T used a similar ploy the last time carriers were tasked with rolling out next-generation mobile data. Then, the company decided to rebrand HSPA+ 3G technology as 4G.

Despite the 5G criticism, AT&T Communication’s CEO, John Donovan, was defiant when he spoke at CES back in January. “I love the fact that we broke our industry’s narrative two days ago,” the CEO said, dismissing the criticism as coming from his “frustrated” competitors.

Responding to the lawsuit, AT&T’s CEO Randall Stephenson told CNBC, “We feel very comfortable with how we’ve characterized the new service that we’re launching,” and that he believed the company is “being very clear with our customers that this is an evolutionary step.”

Stephenson went on to say that he understood why AT&T’s competitors might have been angry about the move, but he appeared to suggest this was because they were incapable of offering the same service.

“It’s not a play everybody can run. It’s a play that we really like, and it’s a play that’s going to differentiate us in the marketplace as we begin to roll this out over the course of this year,” AT&T’s CEO said.

AT&T issued the following statement in response to the lawsuit:

We understand why our competitors don’t like what we are doing, but our customers love it. We introduced 5G Evolution more than two years ago, clearly defining it as an evolutionary step to standards-based 5G. 5G Evolution and the 5GE indicator simply let customers know when their device is in an area where speeds up to twice as fast as standard LTE are available. That’s what 5G Evolution is, and we are delighted to deliver it to our customers. We will fight this lawsuit while continuing to deploy 5G Evolution in addition to standards-based mobile 5G. Customers want and deserve to know when they are getting better speeds. 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 might have been able to wave away previous criticism, but a fully fledged lawsuit might be another matter if the courts are sympathetic to Sprint’s complaint.

Update February 8th, 9:40AM ET: Added response from AT&T’s CEO.

Update February 8th, 11:15AM ET: Added statement from AT&T.