T-Mobile says it won’t stop claiming it has the “fastest” 4G LTE wireless network in its advertising, even after the ad industry’s self-regulatory unit found the claim wasn’t supported. The carrier at first agreed to go along with the recommendation, but then said it found additional, more recent data to back the boast.

The T-Mobile ads in print, on the radio and on television, relied on conclusions from crowd-sourced networking testing studies by Ookla and Open Signal that found the carrier’s customers reported the top average speeds. But competitor Verizon, which has been tabbed fastest by controlled network testing firms like Rootmetrics, complained to the National Advertising Division, the industry’s self-policing body which is overseen by the Council of Better Business Bureaus.

After looking into the complaint, the unit upheld some of T-Mobile’s (TMUS) ad claims, including that the carrier covers 99% of the people that Verizon covers.

But the speed claim was frowned upon. The results of the Ookla tests, which are calculated from millions of people using its Speedtest apps on their phone, could have been influenced by Verizon purposely slowing down data speeds for some customers who used excessive amounts of data in one month, a tactic called depriortization, the division said. Verizon added the policy with its popular unlimited data plans in February.

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“Following its review, NAD concluded that the Ookla and Open Signal Speed test results in the month after Verizon introduced unlimited data plans might have had a bias in favor of T-Mobile and as a result did not support a comparative claim that T-Mobile has the fastest 4G LTE network,” the division said in a statement. “NAD recommended T-Mobile discontinue claims that it has the fastest 4G LTE network.”

The NAD said T-Mobile had agreed to follow the recommendation.

But on Friday T-Mobile said it had additional, more recent testing data now and would not drop the claim. “NAD ruled that the one month of crowd-sourced data we submitted (when Verizon launched their unlimited plan) could not be used,” T-Mobile said in a statement. “NAD previously recognized third-party crowd-sourced data as a way to look at network performance, so we looked at the latest results, and verified what we already knew! T-Mobile is still the fastest LTE network and we’ll continue to let consumers know that!”

Verizon (VZ) praised the original decision. “In our opinion, their claims have been based on cherry-picked data and have misrepresented facts,” a Verizon spokesman said.